According to my count there are 8 avatars, and 9 lost grounds..
My personal theory is that there are as many avatars as there are lost grounds, the last one in my count being Hidden Forbidden Sacrament is the meeting place for all of the avatars, the combination of their sealed weapons kinda gave that away.

But the onlything about this theory that I havent came up with an answer to is which avatar corelates to each of the lost grounds.

Post which order you think they go with

Skeith
Macha
Innis
Magus
Tarvos
Gorre
Corbenick
Fidchell

(Dont have all the lost grounds memorized yet but I think you can figure them out from my descriptions)

Hidden Forbidden~
Battlefield (large dragon in the distance
Waterfall
White tree
Hell
Lava ruins
Fortress in the sky with a stone dragon on top of a building
Wall

If some one could post the actual names for them, I would be greatful.

The Lost Grounds Lists:

(Delta) Hidden Forbidden Radiation = Arche Koeln Waterfall: the waterfall

(Delta) Hidden Forbidden Bulwark = Morrigu Barrow Wall: wall

(Delta) Hidden Forbidden Dead Wood = Dead World of Indieglut Lugh: white tree

(Theta) Hidden Forbidden In Heat = Sugar Mansion Sif Berg: lava ruins

(Theta) Hidden Forbidden Dragonbein = Briona Gwydion the Dragonbein Range: fortress in the sky with a stone dragon on top of a building

(Theta) Hidden Forbidden Battlefield = Coite-Bodher Battlefield: battlefield (large dragon in the distance)

(Theta) Hidden Forbidden Hades = Wailing Capital Wald Uberlisterin: hell


I believe that is all of them O_o, but I remember their being more
though. I'll look them up later XD but what about the Cathedral? What
is that one suppose to be to you?

Hidden Forbidden Sacrament aswell as the place where the forrest of pain is.
Hidden forbidden Holyground to me is a place for one of them aswell, probably being either a representation of either Haseo(Skeith) or Ovan(Corbenick).

If you really wanted to I suppose you could tie in Cubia with the forrest of pain I guess.