DID/DOD Question

Ok, Im hoping you guys can help me out a lil. Im in the process or implementing a new PBX for a location, initially, there were to be no DID/DOD lines for the location, however, they are now requesting some. My question is, how will the number of DID/DOD lines affect the number of T1 lines going to the site. IE. Is there a standard ratio pertaining to how many DID/DOD lines per channel? Any other information you guys have on this would be greatly appreciated as well.

Thanks much.