As many as you like within reason realy. Shrimp don't use much oxygen and also don't make much waste. A group of 10 could be counted as 1 inch of adult fish, so 200 could be held by the tank with no other occupants, though why anybody would add 200 is beyond me

With the occupants, you could add upto 10-20 IMO.
On a side note, the salifin pleco will get too big for a 90l tank. I'm shure you know this though, and will already have plans to move him into a friends tank when he stops growing proberly. He could get as big as 18inches

That said, 15 would be more common
HTH
Rabbut