Can I use API to get from an external network (a server that can connect to Mikrotik) some statistics such as sites being accesses, limits, etc?
Whatever things you can access from CLI, you can access from API. So if you can over CLI, then the answer is “yes, you can use the API for that. That’s what it’s meant for”.
“Sites being accessed” is kind of tricky, unless maybe you’re talking just reading the router’s DNS cache, which is just a single command (answering the question “what”, but not the question “by who”).