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

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 feature Yuki Web service (from 1000 to maximum 5000 calls per day) or Yuki Web service Edtended (5000 to maximum 10.000 calls per day) will have to be added to the contract of the domain.