HMRC rules state that you can open and pay into one ISA of each type per tax year. This includes Stocks & Shares, Cash (including the Help to Buy ISA), and Lifetime ISAs. You’re able to deposit up to £20,000 across your ISAs each tax year, and this limit is split across all of the ISAs that you own.
For example, you could deposit £4,000 into a Lifetime ISA and £16,000 into a Stocks & Shares ISA within the same tax year if you wish!