There is a limit to how much money you can put into an ISA in each tax year (6th of April to 5th of April), which is called the ISA 'annual allowance’.
The ISA allowance for the 2021/22 tax year is £20,000. You can, however, add money to your ISA until 23:59 on the 5th of April each year*, as the allowance doesn’t carry over.
*Deposits made via certain payment methods may take longer to reflect in your ISA. Should you prefer to deposit near the end of the tax year, the execution time of the transaction needs to be considered.