Creation. The Reapers are created by the Catalyst (the overarching antagonist of the original trilogy) who modelled them after the Leviathans. The first one was known as Harbinger. One Reaper was created after one cycle.

What are reapers made out of?

Organic/Geth dreadnoughts are made of metals, plastics, and ceramics. While the codex/various Prothean AIs make reference to the Reapers stripping developed planets of all resources, the implication is that this is done to erase all traces of their actions and keep the true nature of their role hidden.

How many Reapers are made per cycle?

As far as I know, and I would consider myself proficient in the Mass Effect lore, only one reaper gets created per cycle. Now let's assume that most harvested civilizations didn't surrender immediately to the reapers, which doesn't sound like an abstract concept when we look at Javik and the protheans.