13 words
If you input more than 12 words, the next button will grey out. Your seed should have 12.
You cannot use a seed from different wallet (Armory for example or anything else than Electrum).
Nope, 13 is correct for electrum 2 wallets.
Is it? Well my mistake then. I don't really know what is wrong with OP's seed then.
Yeah, Electrum started to implement 13 word seeds in the 2.x versions, however the 1.x versions use 12 word seeds.
OP, what version of Electrum are you using? If you're using a 1.x version you won't be able to recover a 13 word seed, similarly to how you wouldn't be able to recover a 12 word seed with Electrum 2.x.