The demonic career path requires patience, persistence and a little bit of luck. Usually, a fledgling demon will offer itself to a single human as a familiar, disguising their true form such that they appear to be a small domestic animal. Simply by being constantly present in their human master's life, they gain first claim rights to that one human soul. Here is where persistence pays off. If they manage to actually help their human master, despite their limited capabilities, they may earn admiration and even ...love. These emotions, given freely by their human master, greatly enhance the infernal value of that single soul. Feeding off of a passed soul which loved you in life, is like eating Mamma's cooking. It tastes better and is more nutritious because love is packed into every bite.
But all of this is minuscule in scale compared to the obedience of millions which is served to the Demon Lords. Once empowered by a few loving masters, a junior demon might negotiate a better deal for himself, as a demonic servant bound to a master's family. Now he gets to shed his animal form, sharing his true form and through generations of loyal service, becoming loved for who he really is. With multiple generations of masters, each freely offering him gratitude and affinity, he can feast on Mamma's cooking every night, growing in power as the decades pass.
Here is where the luck comes into play. If through the servant's machinations, his human family rises to power among the other humans, that power may be of assistance to more powerful demon servants (or even Demon Lords). The junior demon can barter with his betters, trading his family's services for payment in kind. In this way, a group of demon-served families rise in power, helping each other in a dark and secret cartel. Real earthly power backed by demonic magic is enough to get your junior demon rubbing elbows with greater demons who wouldn't normally give him the time of day. Then it is just a matter of time before one of those greater demons needs a favor in return for full demon status!
The journey up from there is less well charted. Some direct their families into government leadership or ministry, allowing themselves to feed indirectly on the adoration, loyalty and even worship granted to their family members from devoted followers. Others sell their loyalty (and the loyalty of their powerful families) to a greater demon or demon lord in return for an ongoing payment in souls. More subtle and therefore safer is the trading of favors which allows the demon to function with the greater authority than he is really due, granting him indirect influence and abilities rather than actual souls.
The paths which climb down into the demonic heights are many but treacherous. Wise are those demons who know when enough power is enough.