On-line hatch charts are pretty common for the more popular waters all over the country. That will narrow down the number of insects that you're likely to see to a very few.
In a month around here that would be little black stoneflies..... and that's about it.
Jackson River Hatches
I like to tie a minimum of 5 flies of each type
I find that for me that it takes a couple of tries to tie a really good one, but by number 5, I'm getting bored.
Besides if you have 5 and they happen to be the fly of the day, you can afford to lose 3 and still have 1 to fish with and 1 more to bring home to remind you that you need to tie more