According to Micheal Gladwell in his book, Outliers, it takes 10,000 hours of work in a specific area to become an expert at it. If this is true, the amount of things you can become an expert in your life can be expressed by the following:

S_m =  H_w x Y_l x 365 / 10,000
S_m = Subjects mastered
H_w = Average hours awake / day
Y_l = Years lived

Assuming that you're awake for an average of 16 hours /day, the math works out to

S_m =  0.584 x Y_l 

Put into English, you can master over half a subject in a given year if you've spent all your waking hours into it. Assuming you live 80 years, you could learn at most 46 different subjects.

Under a more realistic scenario of 8 hours / day and 60 years (lower than life expectancy but figuring in enfeebled years) lived:

S_m =  0.292 x Y_l 

Which means less than a 1/3 of a subject mastered in a given year with at most 17 different subjects in your lifetime.