He only had a small amount of time to get people.

He lucked out though since they found Kiba walking Akamaru. It seemed like he had Neji, Naruto, and Chouji in mind from the start. Plus Shino, but he is out with his dad on a mission (something I find kind of weird. I thought Gennin only went on missions with their teams)

Plus I think Kishimoto intended 5 on 5 from the start.