As it says on the tin: How many planets are populated in the Ori (ex-Alteran) Galaxy? Is it just the one or is it several?
Non-canon sources are welcome.
Vala also mentions (in SG1: Crusade) that there are potentially hundreds of inhabited world in the Ori Galaxy.
VALA: They are building ships and armies by the tens of thousands. The village I am living in is one of many, on a world among dozens like it.
Since she's evidently getting her info from Tomin (who seems quite a reliable source of general knowledge about the Ori), we really don't have any reason to doubt her.