Many players enjoy mixing their Ascension sets together. Considering each set comes with additional Mystics and Heavy Infantry, it's difficult to determine how many should be included in a single, "mashed up" game.
In short, if you're playing with 2 players, the limit is 30 Mystics and 30 Heavy Infantry. If either pile runs out, you cannot add to it. If you're playing more than 2 players, we recommend including 10 additional Mystics and 10 additional Heavy Infantry per player. In other words, if you were playing a 4 player game, you'd include 50 Mystics and 50 Heavy Infantry.
In the event that you're playing with more than 4 players, capping the total at 50 Mystics and 50 Heavy Infantry should be more than enough to accommodate all players.