I am not quite sure I understand your question. By numbers do you mean extensions or external phone numbers?
Generally each user will have a 2-4 digit internal extension, which can be quickly dialed by anybody in the office. This rings through the PBX and they are generally assigned according to some sort of pattern, ie 2xxx = sales, 3xxx=billing, etc. These are kept short so users won’t have to remember the 7digit number for everybody in the company, plus many extensions won’t need or want a DID, ie the utility phone in the elevator, the hallway phone outside the break room, etc.
Then you have the company main number. This will have simple options (push one for sales, two for billing…) type stuff, and pushing that will usually put you in a queue or ring a handful of extensions to see who picks up.
Often the company main number will allow a caller to directly dial the internal extension of whoever they want if they know it, that way a client who wants to reach their dedicated sales rep can get him quickly for example.
Lastly you have DIDs, personal phone numbers for some users that ring straight to them. Top sales guys for example might have DIDs so callers can call them directly without going through a voice menu. This is optional and many companies skip this entirely.
As for which does what, you may use some or all of the above depending on how they apply to the business. A typical configuration will include the first two, personal DIDs for employees is common but not always done due to the extra cost.
One way to somewhat simplify things- if you can keep all your DIDs inside the same prefix (ie 222-333-xxxx) then you can assign extensions that way. So the guy who’s DID is 222-333-5231 is extension 5231.
As for dialing out, generally you either have the system pattern match ‘external type numbers’ directly or have the user dial 9 first for an outside line…
Hope that helps!