The spell [Phantom Steed][1], 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][2] 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, one in the saddle, the other riding shotgun.

However the spell also says: "For the duration, you or a creature you choose can ride the steed."

How many creatures can ride on the steed?


  [1]: https://www.dndbeyond.com/spells/phantom-steed
  [2]: https://www.dndbeyond.com/monsters/16997-riding-horse