Multiplayer plugin can limit number of players in "rooms". Matchmaking can be a complicated topic, but the simplest would be something like "Join first available room that is not full. If there are none available, create/host a new room". Here is information about how signalling works: https://www.scirra.com/tutorials/892/mu ... pts/page-3
It is a good idea to actually read all the multiplayer tutorials if you are not familiar with them (really mean ALL of them, not just the first one). Don't worry, they are actually much shorter than they seem. You will get a much better understanding of the capabilities of the multiplayer plugin and what is involved in creating an online game with construct.
For what its worth, your game being a top down shooter should be very similar to the ghost shooter multiplayer demo the tutorial covers, so you can be pretty confident that it can be done.