Effect of withdrawing money on ISA contribution allowance

Either the normal annual allowance, or (the normal annual allowance plus £12,000), depending on if your ISA is 'flexible'. From the gov.uk information page (my emphasis):

If your ISA is ‘flexible’, you can take out cash then put it back in during the same tax year without reducing your current year’s allowance. Your provider can tell you if your ISA is flexible.

Example

Your allowance is £20,000 and you put £10,000 into an ISA during the 2020 to 2021 tax year. You then take out £3,000.

The amount you can now put in during the same tax year is:

• £13,000 if your ISA is flexible (the remaining allowance of £10,000 plus the £3,000 you took out)
• £10,000 if your ISA is not flexible (just the remaining allowance)