A - the asterisk that said “a character with a single class”, which implied to me they’d get yet ANOTHER attack if they’ve only 1 class, for instance a pure Fighter
Ignore that completely, what determines how much attacks you can do is your total BAB(normal attacks), not your BAB progression per class. This article regarding the BAB describes it perfectly. That example is using a full BAB progression because the classes that use that can increase their attacks per round faster(Fighter for example).
You can make up to 6 attacks per round when taking in count BAB only, you can make more attacks if you are using other methods to increase attacks per round.
Flurry is a time unit, it doesn’t have to do anything on how much attacks per round you can do. Flurry is essentially when you character does the swing animation or slashing particle(AoO, Cleave, etc), Flurry 1 would be like the beginning of the combat round, Flurry 2 the middle of the combat round and Flurry 3 the end, your attacks per round are split evenly between this 3 flurries.
The maximum number of attacks normally available per flurry by a single character is 4, which can translate up to 12 attacks per round.
- If you are wielding a single weapon, if your BAB is 26+, you can do up to 6 attacks per round which would be splited in 2 attacks per flurry.
- If you are dual-wielding and have the feat “Perfect Two-Weapon Fighting”, if your BAB is 26+, you can do up to 12 attacks per round which would be splited in 4 attacks per flurry. This last point is to what is referring that part that you are mentioning regarding the 12 attacks per round.