The exact thresholds aren't known, but they're pretty low: https://www.awesomedynamic.com/amazon-prime-wardrobe/ mentions thresholds of 30 PPM (and 500 PPM for just Materially Different Complaint Rate) to get into some seller program. Presumably the threshold rate for suspension is an order of magnitude or so higher than that. 500 PPM would be one complaint every 2,000 orders.