How to get number of actors in the party?

For that specific instance I don't care which actors are these, just the count itself. I need to do something only if there is specific amount of the characters in the party.

You can do it thi way using Js:
$gameParty.members() is a method of Game_Party class that returns the array that contains the current party member.
Using length you will check how much of them there are.
You can use the code directly in the conditonal branch script box or store it in a variable and re-use it as you prefer.