The question more appropriately is how many men would want to become top/world class women athletes. And the answer is that it really depends on the sport. Nobody who could play in the NBA is going to want to play in the WNBA, for example, because the money in the NBA is probably better for scrubs than it is for stars in the WNBA. But that does not apply to all sports; top women tennis pros make lots of money, as do top women golfers. And in track and field, a lot of men could get Olympic gold medals in the women's categories, and Olympic gold medals are generally worth quite a bit in endorsements.
Seriously, if you took five men who were ranked in the top 1000* in tennis and gave them wigs and called them women, no real woman would win a tournament ever. The guys would take turns. I was an okay tennis player, never made the boys/men's team in high school or college, but whenever I played any of the women, even the top ones, they were helpless on the court against me. Because of my size (6'2") I could go from side to side in a couple of steps and they needed five or six. I actually felt sorry for them but they wanted me to push them because they at least had a chance on any given point against me unlike the guys on the men's team. The coach of course would be yelling that I was running around my backhand (guilty), but it didn't matter much when my forehand could knock the racquet out of their hands.
*ETA just to clarify I would never have been ranked in the top 1000 so I am not including myself in that group. I played against a lot of guys who were also not in the top 1000 and they waxed me.