Since the FileMaker Go apps will be seen as additional FileMaker clients, I assume that the same limitaiton applies. If you host with FileMaker Pro, you are limited to 9 simultaneous clients.
Yes, the more users you have connected to your PC, the slower it may become, but it's not the most significant factor. The type of actions being performed on the database by each user has far more impact. A single user can slow down reponse times even if there is only one other user if the load they put on the server is heavy enough. Actions that find and sort large numbers of records and reference one or more un-indexed fields can singificantly affect system reponsiveness for all users. It's best to design tables, layouts and scripts to minimize the chances that such a thing can happen in the first place.