I would say 21 BTC but we are talking about retirement which means life becomes a little boring especially if you aren't spending $$$ hoping for the little in your stash to grow while you seat back..for this the number just went up...maybe 100 Bitcoins would do wonders especially if I can have 20 BTC to spend and continue living while waiting for the rest of the coins to do the heavy lifting maybe grow x5 or x10 etc.