The max number of concurrent clients can only be as high as the server parallelism. Assuming each client delivers exactly one stream, there is a 1:1 (client/stream) relationship.
Other limitations (like savegroup parallelism) may also apply.
In theory, there is no limit as more pending backups will just be queued.
If you want to know the max. number of clients - there is none. I have spoken to customers with 1.300 clients. Of course, these could not be backed up at the same time as the absolute maximum number of streams in the data zone is 512.