So some friends asked me to run a one shot for them, they would be playing level 5 characters and there would be 6 of them.  I'm thinking of having them have to deal with a caravan run by monsters.  I don't want to drown them in trash mobs turn 1-2, but also don't want them to just end up in a 1 v 6 brawl against some CR what ever monster with 500+ hp.

How much do other GM's tend to stack the action economy against their players?

Depends.

Action economy is a factor of multiple things, and different party compositions and strategies make action economy very different. If you have, say, a plethora of crowd control casters who can each hold or dominate a target, then action economy can tilt even further against the opponents. I have lost count of the times that I had a creature that just burned through legendary resistances or nat 1'ed saves and the single creature or small group I was running were all controlled. That's not always a bad thing- sometimes handing the players a win matters. However, for a one shot, the stakes tend to be a bit lower, so you can probably justify having a more difficult encounter- being able to just whack things with a stick for five rounds can be boring for both players and DMs.

I would look at your party and see if it would be a problem. Sometimes, you can handle action economy imbalances by setting up encounters cleverly. If you use battlemaps or a VTT, sometimes it is reasonable to have one or two strong front line monsters and then some ranged "trash" that sit in the back, doing fairly limited damage, and then get swept up later. That way you still have a time crunch to take care of the enemies, but you're not just swarming players. Other times, there are other optimal solutions- forcing the party in different directions via an ambush so they can't just form one front, or perhaps making an encounter more complicated (defending other targets forcing the party to fight on the enemies' terms, time limits, environmental hazards that make things more challenging for both the party and the enemies, etc.) so that it's not just an issue of the players acting more.

I generally am comfortable starting encounters with 1.5x the enemies to the party, assuming that makes for a balanced encounter. You can also always give a creature legendary actions, even if they don't have them by default- just make sure it's balanced appropriately (I usually count such a creature as two for balance purposes, unless it has particularly powerful/underwhelming legendary actions) so that you can have "fewer" monsters but still have them matter. Perhaps the troll they're fighting has a legendary action, or some ability that is uniquely challenging that forces them to mind positioning and tactics. As long as the combat is engaging, action economy is less important.

And, finally, being able to resolve actions quickly really does help as well. I ran a session recently with about five times the enemies (swarms) as players. Because they were swarms, it didn't (well, wasn't supposed to, one player just forgot that they had AoE and reduced the party's potential damage output by about 40% but not the end of the world) take forever, since they didn't bottleneck. Additionally, I was running a VTT and since they all used the same stat block I could easily just hit a hotkey to keep rolling their attacks, meaning that every time there were a bunch I could just target and roll, making my time acting as the DM shorter than my players' turns, which is a *very* important thing.

If I balance around action economy, I always aim as much for paying attention to the time it will take me to "play" the monsters as I do to the difficulty of the encounter. I can always modify HP to make creatures last longer or drop faster, but I can't fix a player having to wait an hour because I have a bunch of complicated spell lists I didn't review or tactical decisions I didn't prepare for. As a DM, it's important to make sure that action economy serves the narrative, and to make quick decisions. When players complain about action economy being against them, it's usually as much simply because they sit around doing nothing as it is for balance- that's a separate concern. I aim for a minimum action economy that prevents obvious and ridiculous cheese, but low enough that my players are still the driving force of the session (even if they take fewer actions, I want to make sure that the enemy actions resolve faster and hit slightly less unless they're big bosses).

Action economy doesn't have a magic number, and some DMs are comfortable running a lot (especially if you use a VTT to handle combat quickly). However, what's most important is the flow of gameplay at the table, so balance your encounter with the number of creatures that makes the encounter do what you want it to do without making it bog down turns or make your players feel like you're just throwing wet toilet paper at them.

Now I'm just rambling, but I once had a DM complain that we were killing all his monsters. Well, he was throwing basic skeletons against a party with a monk who did enough damage minimum to one shot them. He was upset because his entire balance for the session was "I'll use 40 basic skeletons and that will challenge this party of level (I think 7?) adventurers." Obviously, that didn't work, but I was just glad that we were burning them down fast enough that the session didn't take ten hours, even if the combat was trivial and meaningless because of the lack of threat and meaningful events. Use an appropriate number of appropriate creatures, less is usually more, and don't be afraid to use tools (multiattack, legendary actions, etc.) to make a few creatures feel like more creatures if it keeps things moving faster.

The mistake I see is trying to use just one monster for this. The reason that's a mistake is using one monster to threaten a group of that size and level you have two outcomes, TPK in two rounds, or the PC's knock it down in one round through coordination.

My fix? Use a bunch of them, heck, have a Mummy Lord with 50 skeletons, 50 zombies, and a Yuan Ti lieutenant and have them buff the skeletons and zombies and use poison area magic that doesn't affect zombies and skeletons. Using monsters in a vacuum means dead monster in two rounds usually. Especially if someone is a battle master fighter and knows what Trip Attack should always be the opener. 

Using a ton of monsters you can always scale hit points down as the encounter progresses and if you take the zombies from 30 hp to 22 hp, i guarantee no one will notice. 

I tend to take what I think of as the 4e approach. Roughly one monster per PC. A tougher monster might be worth 2 or 3 PCs, weaker monsters might be worth 2 for 1 PC. But I will generally aim for roughly equal until I have a good grasp on the party's capabilities, then adjust as needed to maintain a challenge.

1 v 6 is just asking for a stunlocked beatdown in my experience. You need to give a solo monster a whole lot of tricks to take on a party alone.