Precisely how you are using this telephone number is important - as this will influence how many devices can be associated with a specific telephone number. That said, given the number of devices that you are attempting to individually configure, it is perhaps unsurprising that you are encountering a hard-limit.
Given the large estate that you are attempting to manage, you would be nest advised to adopt a completely different approach - and register and manage the estate using Apple School Manager plus an MDM platform of your choice.
Apple School Manager User Guide - Apple Support