You will need to also figure out the rest of it.  You cant say a server will support x phones.  Different phones have different "weights" as do Gateways, H323, Media Resources, MOH, etc.

Run through the sizing tool:

http://tools.cisco.com/cucst/faces/solutionsCatalog.jsp

It will walk you through how many devices your server will support based upon MCS server and devices.

But roughly, any 7845 model can support up to 7500 phones.

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6790/ps5748/ps378/product_data_sheet0900aecd801c1300.html  (example)