There's no easy answer. Some airlines/routes offer no or very limited A availability (e.g. some hot AA domestic routes) but on BA and CX I've generally had little trouble. Not "none", though. And of course it depends on how far in advance you're asking. If it's for tomorrow, it's the luck of the draw. They're treated as revenue seats, though, seemingly way ahead of awards. Look at your favorite booking engine for the airlines/routes you're interested in to get an idea.