Refer to Admin Guide HERE
There is no thing mentioned about the maximum users you can create, but since you already have the device then try configuring it yourself
PS: Since you are that concerned about the credential database I highly recommend that you consider using TACACS+
You have more granular control of the user (what they can and they cannot do). Cisco ACS offers that solution and you can even have hundreds of users configured as well. This is possible because the device polls the TACACS+ server for allowed users. The best part of ACS is that it can cater both RADIUS and TACACS+ spedepending on desired setup you are flexible you can even poll your AD users from it w/c is nice. It will cost you of course, but consider it in your future plans :D
Anyway refer to the guide for more details
For local usernames, it should be able to hold 9 local credentials
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