Vhermano,
Luc is correct, it depends on the maximums for that particular task for the version of vsphere you are running. So, say you have it to deploy 150 VM's it will kick off as many as it can per hosts which I believe is 8 and then it will queue the rest up. So, if you are worried about overloading it with tasks check to see if there is a max operation limit and if so Vsphere will basically handle the load for you.
I don't beleive there is a maximum for concurrent tools upgrades though so it will probably start queing them all up and go as fast as your environment will handle.