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.
If you are married, you can typically claim two exemptions on your tax return.
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
The number of exemptions you should claim on your taxes depends on your personal situation. It is recommended to consult with a tax professional or use the IRS withholding calculator to determine the appropriate number of exemptions for your specific circumstances.
I do not think there is any limit to the number of times you can change a password.