Hi, I;ve got to install cloud core router for 100+ clients for hotel in the sea. From May till September it will be with guests and especially in the weekends where it's going to be full. Maybe I can buy Mikrotik RB3011? Thank you.

It depends on how much bandwidth you have. You can see the test result of routers.

3011 is a good choice.

You also want to take into account the pps of all of those users. You're going to have a lot of demand on that equipment with 100+ users. Now, granted, that also depends on what they are going to use the connection for as well.