How do you maximize the amount of armies you can draft once you have that Tech?

I don't understand why it is very large and then goes down to a very small amount.

The game I'm playing now had a as large as 600 million+ armies and now the last was only 3.5 million.

Did you buy a bunch of mercs? Mercs tend to inflate drafts for some time.

Thanks.

That probably is what is was.