As often as they would like, just fill out new W4.
As many as needed.
You are only supposed to claim the number of qualifying exemptions that you are qualified to claim.
9
9
You can change your twitter name as many times as you'd like.
You can change your religions as many time as you want.
As many times as you want.
6 times
15 times
usually you have to change them at least 4 times a day
I do not think there is any limit to the number of times you can change a password.
Spongebob Squarepants has been employee of the month 26 times and counting