If you haven’t subscribed to Minecraft Realms, i.e. you haven’t bought anything, are you only limited to one Realm you can join or you can join/be invited to multiple Realms?
You can join as many Realms as you want! You only need to pay if you want to OWN a Realm. This can be confirmed with a screenshot from the How do I join a Realm FAQ:
Also, with a quick Google search I was able to find this picture that also confirms that is possible to join multiple Realms: