It varies for each startup. Some may have one tech and one behavioral while some many have two tech and one behavioral interviews. There are sometimes take home assessments as well.
J'ai passé un entretien chez Y Combinator (San Francisco, CA)
Entretien
The first round was a vibe check, and the second round was a take-home assessment. The third interview was a technical. The take-home assessment was average; I could manage to finish in one day.