Hi, I got this question here and I found it is really tricky to solve.

20000 people are watching football match at the stadium. The number of cans of beer that a spectator wants to drink is distributed according to a Poisson distribution with parameter . How many cans of beer do organizer need to stock so that the probability that there is not enough beer is less than 0.05?

I have no idea how to start, can anyone please give me some advice? Thanks a lot.