Really? There are 6 chokes along the left side of the CPU socket and 3 above. Counting chokes is the traditional method of determining how many phases. Did they do something atypical?
If there are more than 8 "phases" on a VRM (such as the 6+3 advertised on the B350 Pro4), it must do something atypical.
No VRM controller ever used on consumer motherboard or GPU supports more than 8 phases. The manufacturer can either put multiple chokes in parallel on each phase (extremely unlikely), use doublers (very likely), or use two different VRM controllers (unlikely).
Buildzoid and his Youtube channel are a great source for VRM information. He does a fairly good job calculating current capabilities of a VRM and noting strange VRM designs.
Edit: Whether or not a 3 phase VRM with doublers is better than a 4 phase, really depends on what the motherboard is being used for. For general usage, the worse voltage regulation shouldn't have any noticeable effect. The additional current capability of the 3 phase VRM is useless for general usage as well.
At the end of the day, this was a marketing decision by Asrock to say "WE HAVE MOAR PHASES" without them actually being able to do anything.