Each tax year, you have a tax-free ISA allowance. You can currently save up to £20,000 this tax year.
You can transfer in any ISAs that you have with other providers too. Transferred ISAs from previous tax years will not count towards your limit of £20,000, but transferred ISAs opened in this tax year will count towards your £20,000 allowance.
Don't forget that this is an annual allowance. If you have an ISA, you can deposit up to the maximum allowance each year, in addition to funds already in your account from previous years.