Is there a maximum number of invoices that can be retrieved through the Sales Webservice feature?

Yes, that number is equal to data retrieval via all other APIs, namely 1000 calls per day. If more than 1000 calls per day have to be retrieved, the Web service feature (from 1000 to maximum 5000 calls per day) or Web service Extended feature (5000 to maximum 10.000 calls per day) will have to be added to the contract of the domain.