A creature can have Mass Polymorph cast on them to gain 157-247 temporary hit points
Each target assumes a beast form of your choice, and you can choose the same form or different ones for each target. The new form can be any beast you have seen whose challenge rating is equal to or less than the target's (or half the target's level, if the target doesn't have a challenge rating) [...]
Each target gains a number of temporary hit points equal to the hit points of its new form [...]
―Mass Polymorph, Xanathar's Guide to Everything, pg. 160
If your DM limits you to only beasts that come from core rule books (i.e. no Campaign books), then a targeted creature could take the form of a Giant Ape [Monster Manual, pg. 323], which would then give the creature 157 temporary hit points―but if your DM allows Campaign book beasts, they could instead take the form of a Sky Leviathan [Plane Shift: Kaladesh, pg. 28] which would give them a whopping 247 temporary hit points.
The only major restrictions are that creatures targeted must be at least CR7 (for the ape; CR10 for the leviathan), or the players targeted must be at least level 14 (for the ape; L20 for the leviathan). Additionally, the creatures will be required to sustain those forms in order to keep those Temporary Hit Points, so they won't be able to sustain those temporary hit points in their normal forms.
- Tomb of Levistus [Xanathar's Guide to Everything, pg. 57] can give a level 20 Warlock up to 200 TempHP in a single shot, although it will immediately drop by however much damage triggered the effect in the first place, so in practice, the maximum TempHP the warlock can sustain with this effect is 199. In addition, the tempHP renders the Warlock incapacitated until it is removed, so while this method can surpass what's possible with Mass Polymorph if the DM doesn't permit Campaign Beasts, it is probably less practical in most situations.
- Symbiotic Entity [Guildmaster's Guide to Ravnica, pg. 26] is a Circle of Spores feature that, if used at level 20, will give the user 80 Temporary Hit Points, and unlike all features mentioned thusfar, don't restrict what the creature can do (except that the tempHP will be lost if they use their Wild Shape again). Since a level 20 druid gets unlimited uses of Wild Shape (which fuels Symbiotic Entity), such a druid could cumulatively protect against up to 80 hit points worth of damage each turn, if they continuously used their Action to maintain the feature and was constantly taking damage, and would quickly surpass the temp hit points that can only be granted a single time per day by Mass Polymorph.
- Tenser's Transformation [Xanathar's Guide to Everything, pg. 168] is the spell that grants the largest possible amount of temporary hit points (50) without involving, ironically given the name, an actual transformation.