Base Camp costs nothing. You get a number that is never reissued, a read of how you actually work, the roster of Climbers who choose to be found, and a library you can take with you.
Your number is assigned when you join, it is never reissued, and it only goes up. Your email keeps the account yours, your name is optional, and your password is stored as a one-way hash. What you write in the journal is yours and is never shown to anyone else.
Making an account means agreeing to the terms and the privacy policy. There is no tolerance here for objectionable content or abusive users, and both pages say exactly what that means and what is stored about you.