Depends on the throughput to your datastore. If you use VMFS (LUN) datastores, I wouldn't reconmmend aligning more than 1 VM per ESX server (as the bandwidth from the service console is very limited). If you have NFS datastores and are aligning from a Linux system (i.e. not the service console) you can probably align 4-8 or more per filer, depending on bandwidth to your filer.
-Michael
