I'm not sure what you mean when you state 10 targets. Can you clarify?
A good way to consider luns are like highways. The more paths "highways" to disk the better your overall performance will be within those luns. If you are planning to have multiple 10 (I/O) transaction occuring at the same time, I would recommend going with 10 luns, but keep in mind there is a max limit on ESX/ESXi.Hope this answers your inquiry
Regards