When selecting to Add Shifts on the Extended Pair option BLS-2021 will keep the opposing Teams on the same shift. This will require using 4 lanes (2 pair) for each match. If the number of Teams isn't divisible by 4 there is a match left over and this match will be assigned to a 3 Shift.
When you select to Add Shifts the program is assuming that the number of lanes can't be increased. So a 10-Team League will still have to fit on 10 Lanes. But if you are allowed to use an extra Lane, there is a way around the third Shift.
Before you set up you Social Distancing option set up a second shift on the Lane Assignment screen. One Shift will have 2 more teams than the other and you can decide which it will be. Set the time for the second shift as well.
Then turn on the Social Distancing, Extended Pair with the Add Lanes option. In a 10-Team League (6 Teams on first Shift, 4 on second Shift), this will put Teams 1 on Lanes 1&2, Team 2 on Lanes 3&4, Team 3 on Lanes 5&6, Team 4 on Lanes 7&8, Team 5 on Lanes 9&10 and Team 6 on Lanes 11&12. In the second Shift, Team 7 will be on Lanes 1&2, Team 8 on Lanes 3&4, Team 9 on Lanes 5&6, and Team 10 on Lanes 7&8.