Quoted from Law:
I believe the rule is that events at undisclosed locations ( not posted publicly via the ifpa listing) that have fewer than 16 players can only occur at the same place three or four times per year.
Eh - it's harsher than I thought:
"There are no limitations to how many events any particular location or organizer can hold. For any tournaments submitted to the IFPA calendar as ‘private’, the IFPA requires a minimum of 16 players to attend for that tournament to be endorsed for WPPR points. The IFPA now requires a full address in the calendar submission, and it is understandable for tournament directors to want to host IFPA endorsed tournaments out of their homes and not have their full address made public. Any private tournament that fails to reach 16 players will not be endorsed for WPPR points."
As written though, a league hosted at private locations would not be subject to this rule, as leagues and tournaments are clearly defined as separate things. Not sure if this is the intent but looks to me like you can have a public league at a private location even if there are only four or five players.