I just tried setting up a cart discount myself with the settings you specified above, and I found that pressing the Submit button to save that discount showed a warning (in red text next to the Submit button) to "Please remove max use count".
Automatically-applied cart-wide discounts can only be limited in use by the min. order total and date-time expiration you set (or whenever you delete the discount, whichever happens first :^), so you would set Max Use Count to 0 (which technically means unlimited uses) for an automatic discount. This is meant to work either like a short-term sale (e.g., "30% off everything through Sunday") or like an ongoing volume discount based on certain minimum purchase thresholds (e.g. "10% off purchases over $50, 15% off purchases over $75, etc.).
The Max Use Count setting is intended to apply to discount codes which act like digital coupons, where a buyer manually types the code into their cart to get that discount, and Merchants might want to limit coupons to be good for only the first X number of customers redeeming that coupon, so setting that number is like issuing only that many coupons to begin with.