Obviously this depends on where you are located. Such as if you live in an expensive place in the US or Canada, then its a lot more than in other countries. Also if you are young, middle age or older.
I always felt the number had to been minimum 10 if you asked me this years ago. But now with news articles and videos saying btc true value is somewhere in the 300-400k range... well that 10 btc number certainly is a bit high. I guess the number probably is minimum 5? Then again you hear old videos where people say... just make sure you have just 1 bitcoin...as that is enough. Thus if it goes to 1 million dollars... well that is a lot of money. Even after taxes, that is a lot. But when people say its half a million or a million, well then that number could drop to say 2 or even 3.
No one knows if it will be able to hit a 300-400k price range it's still a possibility for now. In fact, I don't really plan to use all of my investment in BTC as a retirement plan. As soon as I achieved my desirable market price, I'll definitely sell my BTC and use the money to establish a business or selling only half so that I have an emergency fund.