While there are probably many ways to increase your pool, and maybe even use thing that spawn such as shadows or vampires..... ultimately, a single person can't do this alone.
That is what minions are for. The most obvious method would be simply to start an academy or necromancers or the like and churn out enough to challenge a proper army. Of course, that has the problem where you have to HIDE a secret academy of necromancers (which is hard, since most of the people who agree to join some illegal secret cabal are also the ones that, at some level, are least likely to have the restraint needed for such a cause). Going about this would require a lot of time as you used the most conservative (and often slowest) methods possible. Of course, you would likely aim to be a lich, so time is not exactly a problem.....
Unfortunately, there is no easy answer. Even one of the most powerful necromancers in the setting, the Whispering Tyrant, had to resort to using living orcs as the bulk of his military power. Other compromises, like summoning demon lords/armies, making pacts with dark forces from beyond the stars, and the like are simply the kinds of things you would need to do in order 'win'.
That is what minions are for.
I concur, and would suggest Simulacra. They may cost a fortune to produce, but at least they can be relied upon.
Create undead has no HD limit. Its downside is that the undead you create aren't automatically under your control, and Control Undead does have limits. However, if you use people that are loyal to you already, you can turn them into skeletal champions or juju zombies and they will retain their class, attributes (other than constitution) and memories of their life, and will presumably continue to be loyal to you.