In OFP code "and" and "or" work just like they do in normal language. You want the trigger to fire if any one of the destroyers is in the triggera area, so it would be
(t1 in thislist) or (t2 in thislist)
When trying to figure out what to do in OFP, it's often work isolating and simplifying the problem and making a test missionette to figure out what's going on and what you need to do.
In this particular case you have a lot of destroyers so rather than using a huge stream of or commands you might be better of with something like
Condition: "_x in thislist" count [t1, t2, t3 ...] >= 1
Syntax not guaranteed.
Remember what "thislist" is. It is simply an array of all the units which are in the trigger area and are of the side specified in the first part of the Activation Box. There is no black art or wierd commands associated with thislist. To grab the list from outside the trigger, give the trigger an name and use
list triggerName
Note that this array is updated dynamically. If a relevant unit moves in or out of the trigger area (or dies) then the list is automatically updated.
If you wanted the trigger to fire if all the destroyers were in the trigger you would use "and". That would mean that if one of them was sunk (before it entered the trigger) then the trigger could never fire.