Resellers dont become clients. You can only make a reseller a client if you change the number of sublients to 0 in the settings of the reseller of if you try to assign a client template to the reseller.
limiting databases, mailaccounts, connections ... is absolutely confusing
And what is confising with that? A limit is a number, so limit databases = 5 alows to add the reseller or client to have up to 5 databases... Thats how limits in general and not only in ispconfig are defined. By setting a number that LIMITs the availability of a specific feature.