From your description I'm unsure which limit you are hitting. Each topic can have a maximum of 500 subscribers, perhaps that is what you are seeing? You mentioned 500, but then you mentioned 700, so I was unsure if you are having success between 500 and 700.
What response status code and error description are you receiving from the REST API when connection is rejected?
How many throughput units (TUs) do you have?
Please review article below for Event Grid limitations:
Event Grid limits
Please click Accept Answer if the above was useful.