[Zope] how many zope instances

[Garry Saddington]

I have tried to find information on how many Zope instances can be run on a 
single machine but all searches have failed to be definitive.
Is there any information on this, because I would like to have a separate 
instance for each of my customers running on a different port number and it 
would be useful to be able to predict the number of servers I would need. 
Regards
Garry