I play in two campaigns with the same people.  One group has five players one week and in the other one of the players replaces the DM who is unavailable.  It is not the number of players that is the issue it is how prepared and engaged the players are.  Even with just four players and a single mob, we have one player that when his turn comes up answers us with "Umm what?" He does not bother to pay attention to what is happening on other turns nor what his class can actually accomplish. The number of players matters less then what those players bring to the table. I would play with three more players of the caliber of my other teammates and lose this one guy but you know a friend of a friend ...  

3 or 4. 4 is better if combat is more prevalent, but my own preferences skew away from combat, so I voted 3. More than 4 makes it too difficult to figure out every PC’s arc.

6 players. All the base classes are covered. Some players can switch out playing unusual classes. And if few one or two misses the session the game continues with out a hiccup.

I remember something about Kevin Siembieda running a table for RIFTS on a regular basis with 27 players. WTF

Four.

Four players, first of all, conforms to all the regular game math and makes figuring out encounters easier.  Four players is also enough to give the group some depth without being so many that nobody ever has time for character moments.  If your table likes to weave those moments into everywhere they go (and the good tables do), four players is ideal.  Also makes it much easier for enemies to outnumber the players, which is a very critical part of actually challenging them in combat.  Four players can be outnumbered and still get through combat at a decent clip.

Five is tolerable, though it tends to either result in wallflowers or people getting crimped occasionally.  Add a few HP to a four-man encounter and usually you're good, though you lose a significant chunk of your ability to outnumber the party.

Six sucks.  Too many players, nobody has time for character moments without bogging the game down, and combat is a slog.  You can't generally outnumber the party properly without your combats turning into grindfests, and everybody knows a D&D party never has any trouble taking on anything they heavily outnumber.

Seven and above is just right out.  At that point somebody has to be the bad guy and say "there's just no room in the game anymore, dude.  You're gonna have to find a different table."

I prefer 7+ I love the feel of an epic party.  That being said smaller groups can be very cozy. At the end of the day though we just need to be happy we found any people to play with at all. :) I have been gone for many years and just picked up the hobby again. I have forgotten how much I missed it!  I come from the 2nd edition days. Wow how things have changed. How I wish I had access to something like this  back then. Happy Gaming to All! 

Good topic.

F2F games 4+. Online games 3-5. 

5

Not 6!... 5

I voted for 4, I think it has the perfect balance between a variety and time for the players. 3 PCs work good too, as long as the players choose their classes wisely. 

I played games with 2 PCs and they were cool, those games focus more on roleplay and the PCs backstories, it's very depending on the players if this works or not.

I played with 5 and it was fine too, but the combats were slow. I had experiences with more players (7-8) and I didn't like it. The combat was a slog and the games tend to be less "serious" (players speaking out of character more often, bigger number of jokes, etc.); it was fun but the games felt more like party games. 

I personally prefer more focus on the Story and backgrounds, that's why I prefer smaller parties.

We're playing in a 6-man group right now and it used to be 7. As a player it is very boring at this level. I'm sitting there for ages on my turn just waiting for people to do their actions, re-read what their skills/abilities do and then rolling the dice. It's a drag and we can accomplish a lot more in the game if there were less players, but it's nice to be involved in a large group of people that you're good friends with. It's sorta the upside-downside.

We did lose a player when we began to teeter on to the 6-man scale. Got sick of waiting too long as a monk to simply run up to something and punching it a few times. Plays with another group that has a maximum player limit of 4. I can understand that.

I've been running a two player campaign. They both get a lot of spotlight, but having only two characters made combat really tricky. I had them start playing two characters, which made the combat better but causes the RP to suffer a bit.

4 players I think would be best from an all around balance perspective.

TL;DR I like 5 ish, give or take.

I feel like this bears the price of experience.

The more players you can run successfully depends heavily on how well each player knows the next and how knowledgeable they are about their character and the system they're playing in works. The less of these you have the lower number of PC's work better. If you're group has most of their stuff together DM included then you can trek into the higher numbers without really running into a ton of problems.

There's also the issue of the RP getting stomped on in a lot of ways or personal backstory not getting covered in the long games with bigger parties. However the way we've managed to curtail a small bit of that in the one I'm currently running is have people either have shared backgrounds (two sisters from the same family etc:) or have direct interaction with some part of the story elements where their best interest actually is the other PC's, not necessarily the loots or the mission itself.

I chose 5-6, but 4 is a good one too. A lot of it depends on two factors: (a) how the players get along, and (b) the DM's ability to ensure that all the players get to share and contribute to the gaming experience equally. Sometimes more players can feel like you're in a part of something great, but if there's too many, and you're waiting 45 mins in between your turns...something's gotta change. 

I think anywhere between 4-6 is a good number.  I believe it depends on the players playing. Ive been in sessions where there are 4 people and there’s that one player who asks 5 or more questions at the beginning of their turn and still takes another 5 minutes to think before casting fireball and they just grind the game to a halt. I’ve been in sessions where there are 7 players and it goes lightning fast because everyone prepares in the time between their turn. It depends on the DM to set the pace and the players on how well they prepare. 

It depends entirely on the players and how they interact with each other and the campaign/story.

Quote from JuanSmeeth >>

It depends entirely on the players

Since a few others have stated this, I'd like to explore that notion:

1. I would've though it depended more on the DM
2. Are there truly instances where adding a 6th player (rather than replacing a player) improves the experience?
3. I have three home-groups (and have DM'd hundreds of randomly assembled groups at conventions), but it's always seemed like, regardless of the type of players at the table, 6-player tables would've been more enjoyable for the players had there been 5.

Five - the ideal party-size is actually four, but five allows one person to be away without impacting the game too much.