Each Bold or
Tough ability works. For example, if a creature with Bold 1 got a Weapon that gave it Bold 2, it would flip 3 additional cards when attacking.
There's now an upgrade that says "Opponents cannot have more than Bold 2" or something to that effect. Mechanically, 3 instances of Bold 2 have worked the same as Bold 6. It hasn't been explicitly said that Bold and
Tough stack just like
Pierce, so does it?
[Source: WotC]
Yes. Bold,
Tough,
Pierce, and other key words with numerical values do stack.