unity - How can I specify which enemies to spawn for each round in a tower defense?

I am making a TD game in Unity, and I would like to know the most common approach to spawning enemies each round. I have more than 200 rounds per map, just like Fieldrunners.

Is it some kind of hardcoding, say for 25 rounds and then repeat the same waves with harder enemies, or maybe logic/algorithm based on rounds, where it'll give number of enemies and type of enemies which will spawn in that round.

I have 8 type of enemies. I want to implement a function/equation which inputs round number and outputs "number of enemies" and "enemy type" keeping the difficulty level which is round numbers.

for e.g if input: 50th round, output: enemy type1, large no. of enemies, if the input: 2nd round, output: enemy type 1, less number of enemies