...But we've told you what makes BP "Broken" it's the Synergy of the whole team.
I'm going to stop you there, because when two Pokemon are synergetic, you can always identify how.
Take deosharp for example, deoxys sets hazards, and bisharp keeps them on the field, resulting in decent punishment for switching, or significant punishment for removing them via defog. This is an oversimplification, as elements such as typing and utility moves add things, but that is the most important point.
We can do this with teams too. BP teams have work because they have 7 things; a way to boost spe, def, spd, spa, a way to pass those boosts on, the necessary tools to keep those boosts from being removed by most significant OU threats, and a way to use all to win in the form of a late game sweeper.
Speed is important because it does 3 things; allows def boosts to outspeed attacks so you take significantly less damage from attacks, lets bp outspeed moves such as taunt to avoid being trapped, and let's your sweeper outspeed its victims. Speed can be obtained by either stalling for turns with scolipede, or using agility on something like zapdos.
Defense is important so your chain isn't broken by physical attackers, and can be obtained by iron defense (typically on scolipede) or acid armor.
Special Defense, same thing as defense, but usually obtained from either calm mind or amnesia.
Special attack is important so your sweeper can ohko most enemies. Usually obtained from calm mind.
Obviously baton pass is key to the strategy.
The big player for avoiding getting the
Boosts removed is espeon, being immune to taunt, roar, whirlwind, etc, but mr. mime and sylveon help too.
And the sweeper is usually something like espeon, who can abuse stored power for ridiculous power, and use the passed speed to sweep.
There's nothing wrong with def, spd, or spa, but the spe, bp, and the "immune to checks" thing are all questionable.
Through testing restrictions against scolipede, I've found that teams that rely on agility rely on good play significantly more than scolipede teams, and can therefore be beaten by a skilled player, regardless of the team make up.
Without espeon, taunt and phasing are more reliable, but things like sub and ingrain don't magically disappear. It would definitely be more difficult to pull off a bp chain, but does adding more counters really fix the problem we've been discussing?
BP, is just BP. There's not much I can say to change your mind if you're dead set that bp is uncompetitive, but I will say that without without scolipede or espeon, it's at least not broken. If we do limit it with a cap, I vote 3. 4 can still include pokes to extend the chain and prevent being beaten by common pokes. 2 limits it too much that it's not versatile enough to be viable. With 3, you can have a solid core of scolipede, espeon, and something like sylveon, being able to play around a lot of things, but like any win conditions, there's a couple checks that would have to be left to team members
Sorry I'm slow, and sorry there's no tl;dr version, but I've done a decent amount of work on this, and it bugs me when people act more out of bias or laziness than thinking through the problem and coming to a conclusion that is both logical and effective.