@Easylodges 

Hi, welcome to our community.

I am afraid Deco X50-Outdoor does not support to set speed limit for each device.

And there is no direct one-to-one relationship between the bandwidth used by a device and the number of clients supported by a router.

You may share your network situations like your network network topology, the number of your clients and their types, and so on. 

Then will try to provide further possible suggestions.

Thank you and best regards.