20 Majors will be tough unless things break right for him soon and he gets hot, say Wimbledon and US Open this year. Then with one more to go, I guess anything could happen. It's Roger Federer after all.
# of titles will depend how long he plays and at what level. If he plays to 40, stays injury free, and cherry picks his tournaments, then he has a chance to get a pretty high number. Avg. of 2 titles per year till 40 would be 16. If he plays only to 2016, then do the math.
I guess he'll play till his priorities change, or if his back or something hurts or affects his play to much to continue with enjoyment.
One thing people don't usually look at are Federer's reasons, goals, and motivation that keeps him playing at a high level.
He's accomplished as much or more in tennis than any man alive. How much competitive fire still burns inside of him?
What goals can he set at this point to keep himself motivated? Where does he look for them?
Can he just go out and play, enjoy himself, and try to entertain the crowds that always fill the stands without winning?
If you were him, what goals would you set at this point, to stay at a high level? Win a record 8th Wimbledon, an Open Era record 6th US Open or 5th Australian Open? Win x number of titles before you retire? Win Halle, Basel, or the Masters Cup (WTF) x more times? Eventually, he won't believe he has what it takes to win a major. What will he do then? Keep going, play on and enjoy and win what he can? Or stop, and enjoy other things in his life?
It's not easy to keep the fire burning after accomplishing so much. Look at Andy Murray, OBE. He had a big dream. He dreamed one day of winning Wimbledon, the first British man in over 70 years. He did it, amongst tough competition. And now, it seems to me that he has struggled a bit with motivation, with looking for more goals after fulfilling a dream. Maybe he doesn't need to. Maybe he's satisfied with what he's done. Nothing wrong with that. Very few players in history have won a single major, much less more than 2 majors.
It's the champion's mentality. The desire and fire in the greatest ones to do all they can, what they can, while they can, and then finally end it when that desire dims to a glow that only cruelly reminds them of their greatest days gone by. But perhaps someone like Federer can also play for the love of the game, without winning titles left and right, if that what he wants. It's his choice.
Respectfully,
masterclass