https://www.dndbeyond.com/spells/2304-word-of-recall

Does it cost a spell slot to designate the sanctuary as well as casting a spell slot to recall?

Quote from Blazeynerdy >>

Does it cost a spell slot to designate the sanctuary as well as casting a spell slot to recall?

Yes. You have to cast the spell twice — once to designate the sanctuary, and once to recall — and so you need two spell slots.

However, there's no limit on how much time can pass between those two things happening. And, casting it the second time doesn't remove the designated sanctuary, so if you want to recall to the same place again, you don't need to designate the same place again first.

So now you don't have to choose a location dedicated to your deity as your sanctuary, you can just choose your home base.  This is a good "panic button" spell if you and your party is in grave danger and need to escape.  

Quote from VultureSniper >>

So now you don't have to choose a location dedicated to your deity as your sanctuary, you can just choose your home base.  This is a good "panic button" spell if you and your party is in grave danger and need to escape.  

Or even just a quick way home after a lengthy journey. Especially since unlike, say, Teleport, it doesn't cost anything apart from the spell slot.

Teleport doesn't cost anything either

You're right, I was getting Teleport and Gate mixed up.

It doesn't say the cleric chooses who gets to "teleport", only that up to 5 willing creatures within 5' can. Does this mean that an enemy within 5' can choose to be willing to go along for the ride?

If 7 creatures are within 5' of the caster and they all choose to go (to avoid an incoming high damage AoE), is who gets to go left up to Initiative order?

Can I have multiple designated sanctuaries.

Quote from CruAllen >>

Can I have multiple designated sanctuaries.

Sure, I don't see anything in the spell that says you can't. You do have to cast the spell in each one separately to set them up.

Quote from Seosamh_Fhlannchaidh >>

It doesn't say the cleric chooses who gets to "teleport", only that up to 5 willing creatures within 5' can. Does this mean that an enemy within 5' can choose to be willing to go along for the ride?

If 7 creatures are within 5' of the caster and they all choose to go (to avoid an incoming high damage AoE), is who gets to go left up to Initiative order?

I can't remember where, but I'm fairly certain it states in either the PHB or DMG that when an effect of a player character ability affects multiple things or else has effects where the order it happens is ambiguous, that the player simply gets to decide exactly who or what is affected and in what order effects happen to the target (within the spell or abilities normal restrictions). I would take this to mean that you could simply choose up to the appropriate number of creatures you want to affect, and if, as in your example, the BBEG decides to step up and say hi, you could simply decide that they are the lowest priority in the casting, so they don't receive the ability to be targeted in the first place. It should just say you choose, like almost every other spell like this, but iirc that is the general ruling that would apply here. 

Quote from wycpreest >>

Quote from Seosamh_Fhlannchaidh >>

It doesn't say the cleric chooses who gets to "teleport", only that up to 5 willing creatures within 5' can. Does this mean that an enemy within 5' can choose to be willing to go along for the ride?

If 7 creatures are within 5' of the caster and they all choose to go (to avoid an incoming high damage AoE), is who gets to go left up to Initiative order?

I can't remember where, but I'm fairly certain it states in either the PHB or DMG that when an effect of a player character ability affects multiple things or else has effects where the order it happens is ambiguous, that the player simply gets to decide exactly who or what is affected and in what order effects happen to the target (within the spell or abilities normal restrictions). I would take this to mean that you could simply choose up to the appropriate number of creatures you want to affect, and if, as in your example, the BBEG decides to step up and say hi, you could simply decide that they are the lowest priority in the casting, so they don't receive the ability to be targeted in the first place. It should just say you choose, like almost every other spell like this, but iirc that is the general ruling that would apply here. 

What you're looking for is in the Rules Glossary under "Simultaneous Effects", which is here. It says that if multiple things are happening at the same time, the person whose turn it is gets to decide what order they happen in.

That said, I think what's relevant for this question is that the general rules on spellcasting say that the caster of a spell chooses the targets of a spell unless the spell says otherwise, which this one doesn't, so enemies can't become targets of the spell if the caster doesn't want them to.