Depends on your retirenment idea, how much you are planing to spend, and ofcourse where you live.
From my point, i only need about 200,000 USD value to retire, so that would be about 715 BTC, and 20 years from now mining reward shoul be about 3,25 per block,
resulting in a 6*24*3,25= 468 bitcoins per day minted.
If the money income to bitcoin (judging purely on sustaining current price by paying for newly minted coins) is to remain the same then as it is now (about 1 000 000 USD per day), that would mean that the price should be 2136 usd /btc
with no rise in popularity of bitcoin which is hard to imagine, now just add 10x rise in popularity over 20 years,
and you get 21 360 us per btc.
That would mean, for me, that i need :
-468 bitcoins @ current price
-93,63 bitcoin in 20 years with no increase in popularity
-9,3 bitcoin in 20 years with only 10x popularity (5% growth a year)
Prices im hoping for are something quite different, to be honest, and time ranges also
cheers