The "block" of DID numbers is assigned by your telephone company providing the lines. The extensions receive calls by having their extension numbers matching the DID number. The number of simultaneous inbound DID calls that can be received at one time is the number of DID trunks that you have.
If you print an extension report, you can import it into Excel and then sort by extension number to see how many extensions match DID numbers, to see which have been used and which are still available. Also print out Group Calling, Dial Plan, and Set Up to see in any DIDs are used for Remote Access, Voice Mail, inbound calling groups, etc.
Check with your telephone company to find out your DID block.