I remember when Canada adopted the maple leaf flag, 1965 I think. I have been watching a bunch of WW2 stuff lately, just watched a lengthy D-Day show. I notice the old 48 star US flag in all the films.
But I also recall when both Alaska and Hawaii became states in 1959. I grew up with 48 stars, but Alaska became the 49th state in 1959, and we got a new 49 star flag. it only lasted one year as the Hawaii statehood gave us a 50 star flag a year later.
And yes, there is a 51 star flag ready in case we need it.