If you sell shares you either make a capital gain or loss. If...

If you sell shares you either make a capital gain or loss. If it’s a gain then of course you pay tax. You work out which 20,000 shares you want to include in your tax return - that is, you can pick the buy price for the 20,000 shares from your actual buy prices.

You are better off using the shares with the highest buy price until you get to 20,000 shares bought