First in command - You may only have one Commander per detachment.
Either as Commander rule or as T'au faction rule.
Heros of the T'au'va - T'au sept is free from this rule but has to pay 50% more for each additional Commander (not the weaponry/systems) and they can't be your warlord.
In narrative games you may still take as many as you want.
Now you may throw your stones.
Congratulations, with that nerf the only thing you've done is removed Tau's ability to compete with top tier armies. The answer isn't a flat nerf to commanders. Other choices need to effectively compete with not only internal units, but external ones as well. There's a reason people, myself included, are spamming commanders. It's because outside of two or three situations they are the absolute, hands down most point efficient platform in terms of damage output, and being characters this also affords them the protection that having that keyword entails. A better solution is to make everything else in the book competitive with external units, and the internal balance will come. As I said earlier Commanders do need a slight change. Remove one or two weapon hard points and buff the other suits to BS 3+ and you've not only fixed Commanders, you've fixed the majority of our army issues as well.
Leaving everything else the same just ensures that we'll see even more drone spam with next to zero battlesuits of any kind in sight; which would be weird because battlesuits are THE iconic unit type for Tau. All battlesuits barring the Commander and to a lesser extent stealth suits are in trouble. They are the ones that need a major fix, not Commanders.