What percentage of the Chinese population is Taoism?
It will probably always be impossible to know. In the first place the government is still anti-religion. Even though the temples are allowed to reopen, a census question would never be allowed in China.
Besides, it doesn't really work like that in China. Taoism is not exclusive of religions such as Buddhism and Christianity there. The Chinese will practice all simultaneously. Confucianism is also (though not a religion) part of the blend.
In the past you might chose to practice one in particular if you were living in close proximity to a temple or if it was politically expedient. At heart Taoism is more a philosophy than a religion, but it did develop a hierarchy of "Saints" and rituals. Some Emperors felt that these were more or less supportive of their outlooks during their reign and would pick it as a state religion, and the same might be true of a regional lord.
Peasants however had very few reasons to limit practices that made promises of easing daily worries or offered wonders in the here-after. And priests realized that unless you wished to be a sage there wasn't much reason to be exclusive either, so it became moderately accepted that you used a little of everything.