It is definitely not 10 per day. I heard that it sold out on a few occasions during peak weekends, but they rarely do sell out. Here are some things to consider. First, how close are you to the park and when will you get a chance to go back? If it rains, will you go a different day? To save money last summer, my girlfriend and I bought 4 passes for one weekday in order to save extra in advance even though we were really going two consecutive days - we were told this is okay to do yet not publicized! Second, do you want re-rides? If you are planning on going and you already have the total number of people set in stone, grab a Fast Lane Plus if you want re-rides and less hassle. Sure, you can wait until you get to the park, but it might cost $10 per pass more than bought online before the arrival (we found this true at Kings Island on opening day)!
Note that we had some friends meet up with us for the second day of our trip last year and were able to buy whatever number of Fast Lane passes they liked. But, it was NOT a weekend, so keep that in mind too. You should be good either way, but consider your time, savings, and expectations. Let me know if I can help some more!