Anywhere from 2 to 5 so far. Most commonly 4. Could go much higher- I'd be perfectly comfortable running a group of 8 or 9- but we haven't had that yet.
I have a group of 19 playtesters to playtest both tabletop and online, all under NDAs. I usually run with around 6 at a time, depending who's available at the time i run and on which plateform.
Largest table we got at one time was 9 players, that was less easy to handle!