Hi cgonsalves,
If you run zmprov desc, that command will print all attribute names. If you want to search limit number mailboxes in each domain, you can looking for with run this command
zmprov desc domain | grep -i max
or
zmprov desc domain | grep -i limit
The result of above command, i found attribute zimbraDomainMaxAccounts. May be this attribute is what you looking for. For apply, you can run this command. Example
zmprov md yourdomain.tld zimbraDomainMaxAccounts 10