i won’t claim this is a canonical answer, but I don’t know of any specific limit. I log in from several computers regularly and it doesn’t seem to matter. Certainly you could exhaust server resources and things like that, but I don’t know of anything WordPress specific that would limit it.
I would expect problems if multiple people logged into the same account were editing the same post at the same time. Someone’s edit will get likely get clobbered. If two people logged into different accounts try to edit the same post a warning is displayed. You won’t get that warning if both people are using the same account, at least not last time I checked.
Generally, I would suggest that sharing accounts is not a good idea. Partly, it is not a good idea for the reason mentioned but you are also sharing a username and password among a number of people. Since you are asking “how many” I am assuming you are thinking about sharing it among a large number of people. Security is something to think about. If something goes bad it is going to be hard to find out who is responsible, plus you’d need to distribute a new pass to everyone. And one person could change the pass and cause trouble for everyone. I would advise against sharing an account. Just set up accounts for everyone individually, then you can limit permissions depending on the person if you want.