ITA with gracekelly. A plain tea pot, coffee pot, and cream/sugar set from somewhere like Pottery Barn if you like to play safe, or from an antique store if you have a good eye for that sort of thing would work with the Hermes porcelain you've chosen.
You would probably be well served to go with cups, saucers, and dessert plates to start. Then you could use them either for tea parties or you could use them in a dessert service. As a dessert and coffee service, they wouldn't have to match any dinner service you have, and you could use them for dinner or dessert parties. It's pretty common to have a completely different dessert service than dinner service.
As for number, it really depends on your level of commitment to the pattern. In the secondary market, service for less than 8 is usually the kiss of death, which isn't a problem, if you want to keep it forever.