Thanks for sharing your question with us, Mendy.
Currently, there is no way to put a limit on the number of topics you can assign to each post. However, in the future, we are adding an option for admins, where they can decide which topic(s) should be suggested to users when they are adding the topic to their posts.
Meanwhile, there is a functionality that might help. You can limit the access for generating new topics to admins or moderators only. This way, only admins or moderators are able to add a new topic to the community. The regular members will be able to only pick from the existing topics. You can make these changes form Admin Panel> Settings> Access> Who can create a topic> Save.
