"Usually at least 8" according to the urban dictionary... but then, another definition in the same dictionary suggests that 6 would work...
"Bukkake is when a Mommy and a Daddy and a Daddy and a Daddy and a Daddy and a Daddy and a Daddy all decide that Mommy needs some special facial moisturiser"
http://www.urbandictionary.com/define.php?term=bukakke