The movie never says anything outright about other people abducting Ego's children, but I think it would be almost impossible for Yondu to have done it by himself.
When Nebula and Gamora uncover the skeletons of Ego's children, the sheer number of them is staggering. It's an entire chamber of Ego's subsurface caverns packed head-to-toe with them.
Consider what had to have happened here:
- Ego waits for one of his children to reach a certain age (~7 Earth years, though it may depend on their species).
- Ego pays Yondu to go get the child and return it to him.
- Ego then tries to teach the child to use his Celestial powers, eventually deciding they didn't have any.
- Ego kills that child, then waits for the next one to grow up and repeats.
This means that Yondu was bringing children one at a time to Ego, and there was at least some delay in between trips. It also means that Yondu was continually dropping off children on Ego's surface and never seeing them again. He eventually figured out on his own what was happening, probably based on how shady the whole situation was.
Given how many times this must have happened, I'm not sure Yondu was even alive long enough to make that many trips before he quit doing it. And even if he was, I can't imagine him making it over and over again enough times to pack a cave full of corpses before catching on that something was amiss.
So, if for no other reason than to keep Yondu's intelligence and sanity intact, I think we have to assume there were other couriers before and/or after him.