Someone once told me to do 3 times just like hadachos on meat.
Lubavitchers do 7.
Yosi: Mikvah helps for all other tumos. It isn’t relevant lehalacha for everyone, but even those who have no halakhic reason to do it still like to be tahor.
coffee addict: There are 10 sefiros, not 7.
Shticky Guy: LOL! Some people do 310 (R’ Nachman brings this one down) for a gematria not spoken about in mixed company. I’m sure that for any large gematria there is someone who does it and he will always get there right before you.
BTGuy: I heard that one too but I don’t know the source. Ramchal says that any number is good on erev shabbos as long as you dip the first time to remove tumos of the week and the last to accept shabbos bekdusha uv’tahara.