I'm assuming you wanted the target-address to change, not the net mask on it. Also, you can't use the 'M' and 'k'. Finally, you cannot have a burst-limit less than the max-limit. Burst-limit should be larger than max-limit and you would also need a burst-time and burst-threshold. For example:
:for i from=2 to=256 do={
/queue simple add target-address=("192.168.168.".$i) max-limit=1000000/1000000 \
burst-limit=5000000/5000000 burst-threshold=900000/900000 burst-time=180/180
}
However, I'd suggest you also research the PCC queue type to see if it might be more appropriate for what you might be trying to accomplish.