use the 'loopindex' expression
Repeat 10 times > create object at loopindex*10, 100
will create 10 objects with respective x coords of 0,10,20,30 etc
loopindex can be used for any type of loop, not just Repeat
if you're in multiple loops, you can refer to them (provided they are 'for' loops) like this:
loopindex("loopname")