The number of lines bears no relation to the number of DID numbers.
What you need is an idea of the number of concurrent faxes being received and sent.
Outgoing faxes, on most fax servers, will be queued so you can service quite large numbers of outgoing faxes with a few lines.
To be honest, 24 channels of fax is a large system - but without knowing how many users, faxes per hour and usage profiles etc it is hard to comment further.
You could try speaking to the vendor
Take Care
Matt
If at first you don't succeed, skydiving is not for you.