[Ether]

Since each match generates 2 OPR equations (one for red alliance and one for blue alliance), you need at least T/2 matches to be played (where T is the number of teams) to get an overdetermined system of equations. As you can see from the graphs, it takes a bit more than T/2 to settle down.

But... did you look at the rest of that thread? Eugene Fang came up with a clever idea for getting around this limitation so that a stable OPR can be introduced right at the start of the event... and as matches are played the new data is used to refine the OPR. That new algorithm is used for the match predictions at the TBA site.