Yeah, you need 10 or more to unlock all the CGs from that minigame

The trick I find is to use big sweeping motions across the vegetable (left and right until the cursor switches to a knife, then up and down)... you don't have to be exact, and can overshoot sides of the vegetable with no penalty

Once multiple appear, focus on one at a time, and focus on the one that will "time out" first, because a vegetable timing out will decrease the available time (whereas successfully chopping one will increase the available time)