If Esperanto has such a low number of grammatical rules, is it then a “real” language?

The number of rules says nothing about the complexity of sentences that can be formed. The game chess also has a low number of rules and the board game Go has even fewer rules. No one would claim that these games are therefore “primitive” or not “real” games. As a matter of fact, a low number of rules has significant advantages as this allows you to formulate highly complex sentences, without being inhibited by arbitrary grammatical rules and exceptions.