What will happen if we run out of Bitcoin? I think the supply is 20,000,000 BTC and that we already have more than half of that.
There will be almost 2,099,999,997,690,000 spendable units created.
We already have nearly 75% of that.
The remaining 25% will take more than 100 years to finish mining.
What will we do when BTC stops being mined?
Bitcoin won't exist anymore if the mining process stops. Fortunately it is designed to continue for as long as there are users that want to use it and enough miners to secure it.
Perhaps you are asking what we will do when the block subsidy goes away and there are no new bitcoins being created?
In that case, we will be dead. We won't "do" anything.
Perhaps you are asking what our great-grandchildren will do when the block subsidy goes away and there are no new bitcoins being created?
I'm not sure why you're worried about that. Future generations may have entirely new technologies. That's a bit like someone in 1900 trying to figure out how you are going to handle technology today.
But at a minimum, the mining process will continue and the miners will be rewarded with transaction fees instead of the block subsidy.