Numbered from
what is there.

A new tab is numbered from the tabs you can see

• A restored session took the next untitled number from its saved record rather than from the tabs that came back. A saved file carries an untitled number of its own — unused, never shown — so the first new tab of any restored session was “new 2”.
• A tab the restore could not rebuild counted as well, and left a gap. After 1.1.1 recovered a session whose unsaved tab 1.1 had already discarded, the next new tab was “new 3” with no “new 1” or “new 2” anywhere in the window.
• The number now comes from the untitled tabs actually on screen. Tabs that come back keep the names they had, so nothing is renumbered underneath you.