Question – short-term capital gains tax punishment on stock with multiple purchases

So I know us “poors” (with less than 25k in our portfolio) are punished for selling a stock within a year of investing.

But what I don't understand is how this works if I've bought a stock both more than 1 year ago and less than 1 year ago.

Let's say I bought 20 shares of $VOO over 1 year ago, and then 30 shares of $VOO 6 months ago.

Do I have to wait another 6 months to sell any? Can I sell 20 shares safely? Is there something else I have to consider?

Thanks in advance!