If one is enough for most of us then I think that we're hoping for these prices of Bitcoin to shoot for the next years to come. We don't know when but the time will come for these prices.
It really depends on how much a person spends on their life. Some may think that 5 BTC is enough for them when they retire, while others may need 1 BTC or more. However, if an investor keeps depositing Bitcoin following DCA from the beginning to the end of his career. Then he can definitely own enough money before he retires. Bitcoin creates new ETH in every cycle, in that case, if someone hold Bitcoin regularly for 25 years by making DCA in Bitcoin. Whatever the amount of Bitcoin he has, he will definitely be able to spend his later days smoothly. At that time, it is possible even if 1 Bitcoin is worth 1 million or 10 million.