Most conservatively, I'd read it as saying you can do one pre-scheduled purchase and one one-time purchase per month. But I haven't seen any language saying your one-time purchases are limited. And who's to say you can't keep changing the scheduled date of the "monthly" "automatic" purchase.
A quick experiment informs me that you can't do one one-time purchase after another within the span of 2 minutes. The error message is simply that "This order has already been placed. You may be seeing this page as a result of a double-click or 2 orders placed simultaneously in 2 different browsers." So you may be able to place another order in 10 minutes or 10 hours or who knows.
Update: You can do two one-time credit card orders of the same stock, like, five minutes apart.