I assumed that the first run of creatures will all be from the same base object, but if not then the code below would be needed to keep track of the number killed.
If the point davidlallen brings up is relevant then you can just change the OnLoad to a GameMode block.
I also added the short once in there so the script only executes one time. Could get a crash when that block runs and the second wave of creatures has been cleaned up by a cell reset without it.
float TimeToNextshort countshort onceBegin OnLoadif once == 0if count == 6 ;where 6 is the total number of creatures if TimeToNext < gamedayspassed MyOtherCreature.enable set once to 1endifendifendifEnd
Put a script like this on the first run of creatures
Begin OnDeath set MyActivatorRef.count to MyActivatorRef.count + 1 set MyActivatorRef.TimeToNext to gamegayspassed + 4 :where 4 is the number of days till new creatures respawnEnd