The Eldritch Invocation "Eldritch Smite" says the following:
Prerequisite: 5th level, Pact of the Blade feature
Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra 1d8 force damage to the target, plus another 1d8 per level of the spell slot, and you can knock the target prone if it is Huge or smaller.
Unlike a paladin’s Divine Smite, which says "to a maximum of 5d8", Eldritch Smite says no such thing, so does that mean that a Warlock 5/Sorcerer 15 can use an 8th level spell slot to deal 9d8 damage using Eldritch Smite?
Related: What is the damage dealt by Eldritch Smite?