TDA rule 11-D States what that number should be.
Many times when 2 people late reg for a tournament that has 18 players (or other multiple of the max players at a table) in already they start playing heads up until hands @ other tables finish and players can be moved.
https://www.pokertda.com/view-poker-tda-rules/D: Full-table play will halt on tables 3 or more players short of the table with the most players.
Play halts on other formats (ex: 6-hand and turbos) at TDs discretion.
TDs may waive halting play and waiver is not a misdeal. As the event progresses, at TD’s discretion tables may be more tightly balanced.