This depends on the credit card company and your own circumstances. Companies will look at your financial situation and your credit score to help decide on a credit limit, which is the maximum amount you can transfer to a card.
You will usually be allowed to transfer up to a specific percentage of your credit limit to the card. This may be around 90% to 95%. For example, if your credit limit is £5,000, you may be able to transfer around £4,500. This will leave some credit available to pay for the balance transfer fee and any other charges for transactions that have yet to reach your account.