The spell Phantom Steed, summons a creature that "uses the statistics for a riding horse". The Phantom Steed does have the extra benefit that it has a base speed of 100 feet, and that it does not tire (the latter is not in the spell, but can found in the DMG on p. 243, "Similarly, a phantom steed spell creates a magical mount with a speed of 100 feet that doesn't tire like a real horse.")
Under normal encumberance rules, a riding horse can carry 480 pounds, easily enough for two medium-sized creatures with clothing. So, if needed two medium creatures could ride it, at least for a short time, for example one in the saddle, the other sitting behind or slung over. This also has been confirmed by the Q&A: Are there any rules or rule clarifications regarding multiple characters riding a single mount?. There possibly could be even more (if smaller or lighter) riders.
However the spell also says: "For the duration, you or a creature you choose can ride the steed." You or a creature would only be a single rider. So the spell states the steed behaves like a normal horse, meaning multiple riders are possible, and that only one creature (you or another creature) can ride it.
How many creatures can ride on the steed?