As installed, Virtualmin Pro's Default Server Template has two settings under "Default domain owner limits" for limits on numbers of servers:
"Default limit on number of virtual servers", which is set to "At most 0", and
"Default limit on number of alias servers", which is set to "Unlimited"
When (as root) I go to Administrative Options | Edit Owner Limits for a server that has been created using the Default template, I find these three settings:
"Maximum allowed virtual servers", which is set to "At most 0",
"Maximum allowed alias servers", which is set to "At most *", and
"Maximum allowed non-alias servers", which is set to "Unlimited"
The result of the "*" entry for "Maximum allowed alias servers" is that any attempt to edit the Owner Limits fails unless the "*" is replaced with some number.