how many people see the mona lisa per day?

That is hard to say...If you mean in a global context- as in online, in magazines, print ads, television, textbooks, posters, ect. That number is unimaginable and in all likelyhood uncountable

If you mean at its home, in the Louvre in Paris i can get you a little closer. The most recent count i can find is from 1997 that says attendance at the Louvre is upwards of 25,000 people a day. Although that number is probably higher now, I would say that is a good estament. The Louve is a monumental building and because of this there is really no way that every person that enters is able to see the Mona Lisa. however, it is easily one of the most popular paintings in the museum.

A few millions.

too many to count

A lot.