Trying to find out how many spell slots a warlock could have at a given time. Level 20, access to any magic items that can give spell slots, but only one of each (no duplicates), and straight warlock- no multiclassing.
Four (or 54 depending on how you interpret the Rod of Absorption)
A 20th level warlock has 4 Pact Magic spell slots, and there do not exist any items that give extra spell slots. See here and here for DNDBeyond searches that show all items containing "spell slot" and "spell slots".
That said, the Rod of Absorption can create spell slots within itself, so I'm not sure if that counts:
If you are a spellcaster holding the rod, you can convert energy stored in it into spell slots to cast spells you have prepared or know. You can create spell slots only of a level equal to or lower than your own spell slots, up to a maximum of 5th level. You use the stored levels in place of your slots, but otherwise cast the spell as normal. For example, you can use 3 levels stored in the rod as a 3rd-level spell slot.
The energy is stored in the Rod, so technically the Warlock doesn't have those slots, but it can hold the equivalent of 50 1st level spells.