[(E)]

This is a two part inquiry, the first is how many planes of existence are there potentially in fantasy? The second part is how many planes of existence are explicitly referred to by gurps publications or implied by gurps publications? In general gurps shies away from stating specific planes of existence but some are required to support various spells and powers.

In the first catagory
The four Elemental planes
Morcock style infinite realities.
Hyperspace

GURPS
Hell (or where ever demons come from)
Afterlife realms, where spirits of the dead reside.
Extra dimensional space from bigger on the inside spells