Just to be sure, I would start a few days after Af ends. Yes, you might need more then 7. Remeber that your line has to be as dark or darker then the control line. I found it was best to pee in a cup and then dip the test in that. Good luck!
Oh yea, you want to test at about noon time, everyday at the same time...Remember to hold your pee for a good 4hours before testing.....
To be safe, I'd start on day 10 (day one being the first day of af)...you might want to buy more because sometimes testing once a day isn't enough...I've seen people test in the a.m. and get a 'negative', test in the afternoon and get a positive, and test at night and get a negative...This is why I personally don't like opks. I did much better with the basal body temping and using the fertility friend website....Good luck!
I also peed in a cup to do mine...noon wasn't a reasonable time for me...I'm a teacher so I tried mornings and evenings...
If you cant do it at noonish then later is better, to what i have read.....