Probably 2-3.5, but not more. I expect you guys to support each other and as a DM I will make sure everyone is meaningful in combats and whenever possible extend that policy to noncombat situations as well.
I actually modified your game's setting a bit. Here is what I had and what I have now:
Before
A chaotic evil deity was about to swarm Dis with huge armies in order to take down Dispater and then continue to other Perditions. This is now completely scrapped as I got a better idea and this would really never work out.
Now
Dis is soon to be under assault from a lawful evil entity. What your characters don't know is that the attacker is Lucifer the Satan, Asmodeus' predecessor. He has already ran over most of Avernus and set free the previous ruler of the layer, Astarte, who joined in the fight against the original malefircareim who are still ruling their layers. Belial and Beelzebub are fallen angels, who are yet to declare whether they remain out of the fight, turn against Asmodeus or help their fellow Lords. Due to the events of the Dies Irae, aka Days of Wrath, Dispater and Mephistopheles do not trust Mammon and Lilith is generally very unpopular in Hell and both Dispater and Mephistopheles dispise Leviathan, so their cooperation seems unlikely. However, it is certain that Lucifer has to go through Dis before challenging the other layers.
As you might figure Asmodeus is not surprised at this attack and ever-so-crafty Dispater is hard to take by surprise as well, so they have plans so far-reaching that they might seem preposterously useless. However, they are some of the most cunning entities in the cosmos, so they know what they are doing.
So far Lucifer has recruited Sammael, Astaroth and Astarte to his team. You will not know this yet, but expect it to be revealed at some point. What you know is that Avernus is under a huge attack from an army that doesn't originate from the Seven Heavens or the Abyss, which is pretty much unheard of. Another very surprising matter is that the Lord of the First, Bael, seems to be utterly outmatched and he has called for back up. How the other Lords answer to this will tell a great deal about the future allies and foes of Dispater...
History of Hell
Both Lucifer and Asmodeus are avatars of the true Overlord of Hell, who again was a part of a trinity that formed the axis of law, The Progenitor of Nirvana, The Supreme Virtue and our little Overlord. Lucifer went AWOL after a battle with Asmodeus around the Dawn of Gods when first deities started to emerge countless of millennia ago. During the early millennia of Lucifer's reign the evil energies of the Nine Hells of Perdition formed the first malefircareim who we know as Astaroth, Dispater, Mammon, Adrammelek, Leviathan, Lilith, Sammael, and Mephistopheles. These eight also became the first Lords of the Nine in the court of Lucifer the Satan. Actually, some of the Lords were already very close of controlling an entire layer before Lucifer came in, for example Leviathan just swore loyalty to Lucifer when he first saw him even though Stygia was in his hands already.