Is when I buy a template Regular License, I can create a website for my one client?
In that case, do I have to transfer the license for it? How to do it?
Yes, you can purchase the license on behalf of your customer but you cannot transfer the license.In that way, I suggest you to create new account for each customer and when you’re done, share the username/password credentials as well
And that if no create customer separate account it will all be legal?
It’d be still fine but you’ll be holding the license, not your customer.
You can “transfer” the license for the client to use the item (assuming you then remove all copies etc. from your possession) from but as @ki-themes said it’s not as good as having them buy the item themselves in the first place as updates and the ability to download the item will only ever be from the original purchase account.