Plus the Baseline zone, which is required. So that gives a total possible BlueValues entries of 14, or 7 zones.
OtherBlues is an optional entry, so the Baseline zone should not be included when counting the number of potential zones, to avoid confusion. The maximum for OtherBlues is 10 pairs, or 5 zones, one or more of which can actually affect glyphs above the baseline.
Glyphs apparently has a way to determine which numbers go where (BlueValues or OtherBlues) based on the outlines. If not, then a Custom Parameter entry would be needed so the user can add them.