Hi 613TX,
If you invest £20,000 in the tax year, 6 April 24 to 5 April 2025 you will need to wait until the next tax year 6 Arril 2025 to 5 April 2026 to invest a further £20,000 in any ISA account.
Please have a look at the guidance on ISAs here:
Individual Savings Accounts (ISAs)
Thank you.