I am expecting a large bonus. How many exemptions should I claim on my W-4?

We were told by someone to change his w-4 before he gets a large bonus and then change it back after he receives his bonus? How many exemptions is reasonable to claim? We both work FT ( he has two jobs technically) and we have one son and a mortgage. Also, he's military and returned from deployment this year and we feel after what he went through while deployed, he deserves as much of his bonus as possible. Thanks for helping.