Dear, sorry for my bad english and sorry if the question is not totally about your product.
Is there a maximum limit of custom fields that can be added in joomla?
I'm writing all the fields to be able to use them with the Articles Good Search module with connected fields but now it does not save them anymore, it seems that i reach a limit.
I entered about 500 custom fields, is there a maximum limit?
It should be no such restriction in Joomla, but it can be some issue on server side. Please, try to enable Joomla debug mode and error reporting to maximum, then you should get some error.
Also, it can be database limit, if you have some huge textarea field, then it will not saved. You can change field type in the database for avoid it.
E.g. _fields_values table and 'value' field: change type from 'text' to 'mediumtext'.