Multiple Blackjacks on a single deal.

I've seen the odds of getting blackjack are about 21/1. But how can you calculate the odds of multiple blackjacks on the same dealt hand? For example last night me and a buddy where playing on a six deck shoe with two other players, we were all dealt blackjack and the dealer was showing an ace. None of us took even money and sure enough the dealer had twenty-one. What are the odds of that and how would you calculate it? What would be the odds if there were five or six players that were all dealt blackjacks? Thanks!