Alright then, for numbers, you can scale yourself however you want pretty much within reason. Playing an understrength faction can be fun afterall. Keep in mind though that this is 40k, most Imperial Guard regiments fielded a company of super heavy class tanks and hive worlds often have populations of over 100 billion.

I had planned to have a few examples of basic tier NPC nations hanging out but work got in the way, (I have worked 76 hours in the last 7 days).

To kind of think of the scale, since hive worlds will often go over 200 billion people and most forge worlds have their own titan legions and the most common titan is the warlord. Hive worlds I figure average 100 billion people in their population. If you put .05% of that into Imperial Guard regiments using local industry to arm the with lasguns and APCs that gives you 50,000,000 mechanized infantry. This being 40k you can easily support more than that. If you have a world with a bit more industry, either a hive ot civilized world, you are looking at hundreds of thousands of tanks. Super heavies and titans cannot really be built domestically without a forge but while a forge world seems to average around 10 billion people that is still easily hundreds of super heavies. 

As i said, the main limiter on battles is just how many troops you can bring to a battle with your fleet.