A Stocks & Shares ISA is a tax-efficient investment account. This means you don’t have to pay UK income tax or capital gains tax on money you earn from your investments made through the ISA. However, you're limited to investing up to £20,000 per tax year across all of your ISA accounts.
The tax year begins on April 6th of the current calendar year and ends on April 5th of the next calendar year. During this period, you can invest up to £20,000. Once a new tax year begins, your allowance resets, providing the ability to invest £20,000 more.
As an example, if you deposit £15,000 in your Stocks & Shares ISA and then withdraw this £15,000, your remaining available balance will still be £5.000 as the withdrawals are not adjusting your allowance towards the £20,000.
Accordingly, if you reach the annual limit of £20,000, you won't be able to make more deposits, even if you have withdrawn some of the money. You will have to wait for April 6th, when it's the new tax year, and you get a fresh new allowance.