If you hit sell, Freetrade will sell VUSA and the cash received from the sale will stay in the ISA account. And if it stays in the ISA account, it stays tax free. So you can buy and sell as often as you like inside the ISA account.
The 20k refers to the āsubscriptionā - the amount of cash you deposit, not the value of stocks you later buy with that cash.
Full ISA rules here: Individual Savings Accounts (ISAs): Overview - GOV.UK