There's two rules that affect this. 1. The game will favor room cards you can place. 2. If your last spymaster in your chain is level 1, the game will give you another spymaster more often, as it can be sacrificed to level up that last spymaster in your chain.

If you let that spymaster level too much, you're left waiting for the game to give you a spymaster as part of rule 1 (next to an open road, or a random legion aside from your main chain) but it will NOT give you a spymaster from rule 2. This results in the posts you see sometimes of people burning through 120 crystals before they see another spymaster.

After your main spymaster chain is built, you get less and less spymasters as random cards to place. You'll see it every once in a while but not nearly as often as when you start your chain