I haven't seen him use more than one. He was able to use the doctor's power. Then he grabbed the quick guy and now he has that power. Where do you see him being able to have more than one?

Season 1 when he learned to pull out any power he wanted. He didn't have to be around the person to use the power once he learned to control it. He just needed to be near them to grab it. Once he got it he kept it and could pull it out at any time.

Hence how he could become invisible, move things with his mind, have super strength and stop time. Then in season 3 his dad stole his ability leaving him normal. At the end of Volume 3 he injected himself with the last dose of the serum and he got powers back but he couldn't use all of the powers he grabbed. He could only use the last power he grabbed. So if he was near Hiro he would ONLY be able to stop time. But if he got near Nathan he would lose his power over time and would only be able to fly.

Now he seems to be able to do what he could do in season 1. Keep all of the powers he grabs and use them whenever he wants.