When supply reaches zero and demand is a positive number, Bitcoin’s price is infinite.
But supply cannot reach 0. First of all, it will take decades before we reach 21 million, so you don't have to worry about it running out. Maybe your children will face this problem if Bitcoin exists in their age. As long as there are buyers there will be sellers. For every price level there's a seller. They're here now at 30k and there will be some at 300k USD.
It does take a very long time to reach the maximum supply limit, but you must know that when the supply becomes maximal it will make bitcoin prices more expensive while demand may increase, therefore buy bitcoin at a price that is still cheap as it is today and save it for investment long term could be the best choice.