I don't think theres a 'magic number'.
The most important is the console's sales momentum. It really doesn't matter if the PS3 is selling poorly, but has momentum and keeps increasing and getting stronger. This is important, because it gives developers hope that with new console addopters (at a reasonable rate), they will pick up their game.
Likewise, it's not a great decision for a software developer to put a game out on a strong user base, but failing sales-type console (think N64 and Dreamcast in their later years). Even if they have a good install base (N64), if sales are down, gamers are probably disinterested in that system, and will fail to purchase alot of games for that system. Look at the PS2 in Japan. It does have a few great-selling games, but losses to the DS by about 3:1 YTD in terms of sales. This is important, because the PS2 not only has the higher install base, but the higher software attach ratio, and is still selling ~15,000 units a week.
Back from the dead, I'm afraid.
