@Pexers Thanks for reaching out. Can you please confirm if all your 100 function app is running on the consumption plan? In case if it the same consumption plan then there might be case where there may be high resource utilization at the same second for the same consumption instance which yields to 503 error during that time there the general recommendation is for each function app to have its own Consumption plan.
Yes, there is limit of 100 consumption plan per region as documented here.
You can refer to Diagnose and solve problems blade to get more details/insights what would have caused the 503 errors.