As Scaley says, if you are buying Bromptons the most number of gears you can have is six, achieved by a three-speed hub, two sprockets and a derailleur.
Of more importance will be your choice of range, Brompton offer low, standard and high gear ranges - all that's changed is the front sprocket.
I bought standard, but found it too high, so changed to low.
Even with the low range, spinning up 6th gear takes some doing and produces a speed which I reckon is more than fast enough for travelling on a Brompton.