Just today I finally got my 34th ally! Some of those don't even play anymore, and some are lower level players that I allied to lend a hand. 35 seems like plenty to me.
My point was that at 35, you can't even be allies with everyone on your guild. I like DevTore's idea of making everyone on your guild auto-allied and having the other 35 ally spots open to people not in your guild.
The only problem with the idea that your guildmates should be your allies is what happens when you withdraw from/get kicked out of/disbands the guild? You then have 0 allies. I really don't think there's any compelling reason that your guildmates have to be your allies but they certainly represent a good population to draw allies from.
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It should be more than 50, I dont want to be forced into a situation that my allies can only be my guildmates.
A better solution would be to have everyone in your guild automatically allied, and on top of that have the current 35 available for external allies.