My oldest probably had 8 pair when he was potty training.
My second has caught up to him in size, so he can wear his undies, or his brother's. And he uses them all up, all the time! It's so frustrating! I've given in several times and bought more, but it doesn't matter, he's always out.
My oldest passed on all of his undies to his brother, all in great shape.
The second trashed them all until they practically fell apart.
Each kid is different. I think it's funny that you bought 30, but mostly because I think the difference between kids is funny, not because I think you overbought. Mom knows best!