Considering that not all classes are full and not all that show up make it through, I'd say the answer is "as many as we can get." Don't have to worry about them maxing out the pay scale, or probably going somewhere else in six months.

When I was in training a couple weeks ago there was a retired ATC Controller in new hire class, and I know of one FO here who is at least 60, is retired military, was a CA at ATA and original Midway, and was furloughed from Eagle a couple years ago.