There are more to come. Vulpera are pretty much a given at this point, and I expect them and their Alliance counterpart to be announced at Blizzcon. And after that, possibly another pair later down the line.
BFA IS the expansion where you can expect a regular cadence of Allied races to keep trundling off the conveyor belt.
8.0 Mag'har and Dark Irons
8.1 Zandalari and Kul Tirans
8.2 Vulpera and ?
8.3 ??
See how it follows the traditional patch structure for a WoW expansion now?
At launch we get a raid, we get a new a warfront and two new allied races.
Then each major patch will bring a new raid, a new warfront and two more allied races until 8.3 when the Horde wins the war and saves azeroth from the old gods
.
After that I expect the number of allied races introduced to fall drastically to a pair each expansion maximum, with no guarantee of a pair at all.