I think 2 BTC won't be enough. Let's say 10 BTC to be so sure.
Since we are talking about here is the retirement and I know what you mean is you are going to rely all your needs on bitcoin that why it would be better if you would going to buy 10 BTC and holds it for your retirement. You're lucky if the price of the bitcoin when that time comes would be about $1,000,000 per BTC, I'm sure 10 BTC is too much for you to spend it alone.