There are 18, 9 of them are not in the PHB, and 3 are available only through Pact of the Tome.
This DnDBeyond search shows several on the Warlock spell list:
- Create bonfire (cantrip, XGtE)
- Green-flame blade (cantrip, SCAG & TCoE)
- Hellish rebuke (1st level, PHB)
- Spirit shroud (3rd level, TCoE)
- Elemental bane (4th level, XGtE)
- Sickening radiance (4th level, XGtE)
- Wall of light (5th level, XGtE)
- Investiture of flame (6th level, XGtE)
- Crown of stars (7th level, XGtE)
And on the Celestial Warlock's expanded list:
- Sacred flame (cantrip, PHB)
- Guiding bolt (1st level, PHB)
- Flaming sphere (2nd level, PHB)
- Wall of fire (4th level, PHB)
- Guardian of faith (4th level, PHB)
- Flame strike (5th level, PHB)
Notably, there is only one spell on the generic warlock spell list in the Player's Handbook. Eight more spells are added in Xanathar's Guide to Everything, and Tasha's Cauldron of Everything.
Available only through Pact of the Tome:
- Fire bolt (cantrip, PHB)
- Produce flame (cantrip, PHB)
- Word of radiance (cantrip, XGtE)