On the RPTQ fact sheet, the number of invitation starts at 2 (for events with 42 players or less), but the prizing had a column for events with only one invitation.

Is there a threshold below 42?

This seems to cover it for you:
https://magic.wizards.com/en/articles/archive/organized-play/grand-prix-updates-2018-promos-schedules-and-day-two-cut-2017-10-26

d:^D