I have 400 quotas and if I add one more I'm getting an error
'Maximum number of Resources for this API has been reached.'
What is the maximum number? 500-800? I want to know if I can extend it for another 200-300 quotas or I need to create another API, thank you!
As per the documentation, the default quota for Resources per API is 300. Reviewing the documentation further we can see that this limit can be increased which I would suspect has already occurred on your account.
If you would like to increase this further, you can use the console again and request a service increase, a useful guide for this is here.
As for the upper limit, this is not listed and most likely wont be listed as it will be at the AWS service teams discretion to do so. Based on my experience, you can usually get 100-150% more than the default quotas just by requesting a service increase in the console. If you would like more than this you may have to create a support case and give justification for the request, but, as long as it is reasonable, it will usually be accepted.