There are a few ways to estimate the number of tourists in Paris. One quick way is highlighted below.
Take the total population of Paris - ~10M
Assume on average in any given day ~2.5% of the population are some form of tourists (this includes domestic, int'l, EU, business etc.) - so 250K people on any given day
Assume that on avg, a tourist stays in Paris for maybe 3 days (Some more and some less)
That means every 3 days there is a new set of 250K people coming into Paris. Which means in one year, 122 sets of 250K tourists visit Paris (365/3)
122*250K is ~30M tourists annually in Paris.
You can play with these assumptions to get a different number
Hope this helps,