that is easy ,after 50 people you are at 0% .

For this, imagine that VP regeneration was turned off. Technically its near impossible to get it down to 0% because your vote value would be too low(not even dust) and so the system wouldn't accept it. Also we are assuming that you are doing 100% votes to reduce VP at the maximum possible rate.

Every vote that you do takes 2% of the VP that you have. So the first vote would take 2% because 2% of 100 is 2%. Now the second vote isn't as simple. Because you lost 2% from the first one, you will only have 98% left. 2% of 98 is 1.96% so you will have 96.04% left. If you keep that going after 227 votes, you will be under 1%. Every vote will get you closer to 0% but you'll never truly reach it. Steemd shows VP down to two digits and it would take 455 votes to bring it down to 0.01%. 

So the answer is that it would take an infinite amount of votes to get your account down to 0%.

You can see how much you'll have left after each vote on this sheet. But keep in mind that you'll regenerate that back and after a point, you'll be regenerating more than you'll be spending at 100% votes.
https://docs.google.com/spreadsheets/d/1E5l_bZtvyNk5oqxzN84yiRIdtOXnrBqwIrhnLX26OBg/edit?usp=sharing