The maximum Medal value is 999999.
How To Get Medals
Currently are know 5 ways to get Medal:
The most basic one is buying Medals with a credit card (Price vary by country).
Another way is limited but free, fast and relatively easy, and it is done by clearing the campaign level known as "Sortie". (Starting from recent updates, when you achieved S-rank on all levels in an area, you get 3 Medals). The total amount of medals that you can get in "Sortie" after finishing ALL areas with S-rank achieved is about 700+ Medals.
Event areas are also a source of medals, though the amount of Medals awarded varies per event.
A third way is more long in terms of time but it's also the easiest way, this way is the Login Bonus. It will only get you a few Medals, but with enough patience, you can purchase Medal-based units that you want.
The last way is the newest one, and is by completing Missions in the Missions Tab. Each 5 mission will award you 15 Medals. For the first 10 missions, it will award you 30 Medals.
Known Prices of Medals
United States Prices: