If the Army you control leaves the battlefield with another amass spell or ability on the stack, it simply creates a new Army token
Theres a lot of established heuristics on this subject, and a few points I can toss in to help you construct an optimal mana base for Standard, Modern, Pioneer, or whatever MTG format you prefer
Later on, it loses a lot of its potential, but hopefully you can just trade it off and be done with the ability
If you can surgically extract artifacts from your graveyard one at a time, perhaps with unearth or a single-target exile effect, you can flood the board with artifact creatures in no time
#6 I'll be happy to look at the art when it's revealed and go back to pretending these don't exist because I've never seen one for anything less than double MSRP #7 One of the Packs is literally pitch black
Given how prevalent scrying and surveil mechanics are for the color, being able to draw after every single activation is an excellent way to get even more value from great cards