I am at the moment running a poll on LinkedIn exactly dealing with your question.
In fact, the question is vast, mainly depending on the complexity of the instances.
I have recensed several customers with more that 180 instances for 0 DBA ( in fact one didn't even remember they had informix instances, but all their distribution centers are relying on Informix since 1993...)
I have also recensed a good number of customers who have heavy and complex informix infrastructure, where 1 DBA runs between 16-30 instances.
I guess if you have 1500 instances, they must be pretty much similar in design. If you switch to 11.70, DBA requirements are much less since more and more DBA tasks can be fully automated ( this is why Cisco chose Informix for their telephone systems )
I invite anybody concerned by those figures to fulfill the poll on linkedin, where you also can see the results:
here is the url