There are a few ways you can count the remaining players, such as counting once every x seconds, or counting only when someone dies.
Below, I have provided instructions on how to count the remaining players every time someone dies. The code assumes that all players belong to the same group. If they do not, then modify the code as necessary.
1. In the group leader's initialization field, put
squad1 = group this. This gives a name to the group.
2. Also in the leader's initialization field, put
squad1 exec "CountPlayers.sqs".
3. Create the CountPlayers.sqs script with the following code:
; Take the passed group name and assign it to the _group local variable.
_group = _this
; Initialize the _OldCount variable by giving it a value
; of the number of currently alive players. This code assumes that the
; players belong to the West side. _OldCount is used to contain the
; previous number of alive players for comparison purposes.
_OldCount = west countSide (units _group)
#Loop
; Wait until the current number of alive players and _OldCount are not
; equal, which indicates that someone has died.
@ (_CurCount = west countSide (units _group)) != _OldCount
; Display a hint indicating how many players are still alive.
hint format ["There are %1 players remaining.",_CurCount]
; Set the old count to the current count.
_OldCount = _CurCount
; Loop the script.
goto "Loop"
Good luck.