22.
Mnn... hard one, since I like all of 'em. ALMATERIA is out of the question because it's impossible to translate. Soo... Tenkuu no Canaria, and Ta ga tame no Sekai, since I'm not certain if TETHE'ALLA ~ Tai no Kodomatachi ~ has been translated yet or if it even can be. Might be in the angelic language like ALMATERIA.
Lyrics wise I think your best bet is Tenkuu no Canaria. They're the easiest to obtain, though you can also try Inori no Kanata. The two songs are Tethe'alla's opening and ending, so I would think those would make the most sense to use, and no one has put Ta ga tame no Sekai's on a website yet.