RoundCube isn't a mail administration software package. It's just a way to read your email.
If the mailbox itself doesn't have the limitation, then there's no way to limit the number of messages sent / received / saved by the person as the limit would be arbitrary.
For example, if RC imposed a limit of 500 messages, and only listed 500 messages from 1 to 500 (no skips), and someone sent me a message, it would be message 501. The mailbox would show as "unread", and RC would see it as "unread" from the server and would mark it as such but since there's the 500 message limit by RC, it doesn't "see" the 501st message. So when you delete say number 499, you've all the sudden got a new message.
