36. Running the masking

Enabling “Use number of rows from system statistics table” option significantly speeds up row count calculation, yet relies heavily on the accuracy of your database's statistics tables.
It's strongly advised to confirm that these tables are up-to-date before proceeding. You may refer to The number of rows in a table using table statistics for detailed instructions on refreshing them.

You can read the detailed explanation on how parallel update of single table works here: 36. Parallel masking of a single table