What's your time frame of LORs? I assume because each school has specific forms/questions, you must wait until apps are available to pass along LOR forms to recommenders?
Is it wise to approach potential recommenders now and give them a heads up about LORs towards the end of the summer?
How many recommenders is a good number? Assuming you are applying to 6-8 schools, you wouldn't want two people filling out all LOR forms...
1. Yeah, you need to wait until the apps are available as each school has its own style. Though I rather doubt the school changes the form from one year to the next, but it's probably better to be safe and wait.
2. That really depends on your situation, and was much-discussed here last year. My work is a cool place and I'd put my time in, so I basically gave them 9 months' notice and had almost no repercussions. (they were less keen on sending me to trainings as the end approached, but I don't blame 'em). For other applicants, they never told their bosses until they gave their resignation. You need to figure out how it's going to play out for you. But generally, the sooner you can tell your recommenders, the better.
3. I've heard of people doing this both ways. I had 3 professional recommenders (and one peer) for 5 schools, just to even out the burden. For the most part, writing the first letter is the hardest. After that, the recommenders can adapt the first letter to any subsequent ones. (But I should say that some schools ask very specific questions that require specific answers while others are looking for a more general letter.)
That's my take.