Is 8 a low number for the result of a day-21 progesterone test if i'm trying to conceive w/o success

Yes: Generally, we like to see a day 21 Progesterone over 10. Any number above 10 means that you probably ovulated. If the number is less than 10, it would be helpful to check a day 3 FSH level. Also, ovulation predictor kits and ovarian follicle size checks by ultrasound can be helpful. Workups vary from expert to expert and there are many "right" ways to proceed.

Progesterone varies: Progesterone levels bounce around throughout the day, depending on day of the cycle and pituitary stimulating hormones. While a Progesterone level of 8 is on the low side, it is usually enough to support a pregnancy, and might well have been higher if retested a few hours later. We don't find Progesterone levels to be very helpful in planning fertility treatment.