A project did not save
The project cap, private browsing, and what the save label was telling you.
Read the label first
Under the Save button is the state: unsaved changes while you are editing, Saving while a write is in flight, and Saved when it is done. Saving stays up for a moment even when the write takes no time, so a save that worked is still something you saw.
If the label went red or the editor put a message up, it says which of the following it was.
You are at the free plan's cap
The free plan keeps ten projects. The message names the number and the way out: delete one, or move to Pro for as many as you like. Students pay half with a school email address.
Your account page shows where you are, for example "7 of 10 projects", so you can see it coming.
Your session expired
The account refused the write. That is not something you can do anything about, so the work goes to this browser instead and the editor says where it went. Sign in again and press Save; the document is still open in front of you.
A new project is only created when you press Save
Autosave to the account only ever writes a project the account already has. A brand new document is created on the account when you press Save, never behind your back. If you were expecting a project to appear in your account list and it did not, press Save once.
Signed out, Save writes to this browser's shelf of projects, and the Save menu's Save to my account is the one press that moves it up.
Private browsing, or blocked site data
Everything signed out is kept in the browser's own storage. In a private window, or with site data blocked or cleared, that storage can be empty or refuse to be written, and the editor carries on rather than failing. Nothing is lost while the tab is open, but closing it will lose it.
If that is your situation, sign in, or use Share to get a link that carries the whole project inside it.
You are watching someone else's project
Pressing Save there forks it into a project of your own instead of refusing. The fork is named by the server. See sharing a project.
How many versions are kept
Two hundred per project on every plan. Autosaves that land within a minute of each other share one version, up to ten minutes of editing, and every Save and restore keeps one of its own, so a long editing session does not push out a version you wanted from yesterday unless you made two hundred of those since. The free plan opens the newest twenty; the rest are kept with a padlock on them, and Pro opens them all.