Depending on your variety and seed packet, eggplants and some hot peppers are sowed between 8-10 weeks. Last year these were slow starters for me, and although I have fresh seed for several of them I went ahead and started most of them on the early side of the range.
As these are heat-loving plants, and our house is at a chilly 60-something degrees, I wrestled my heat mat away from my home-brewing husband and put it under the flats to help with germination. As I'm cheap and only have one small mat, I stack flats, and rotate plants out as they sprout. Our bathroom is also for some reason the warmest room in the house (plus bonus humidity!) so I've used that in the past-- there are lots of other low-tech options for boosting heat.
While it can be a little torturous to start thinking about preserving before your seeds have even sprouted, I find this is a good, realistic time of year to assess what you've been eating. At the beginning of winter, you are using up fresh stores, and possibly hoarding some of your more precious preserves. But by March, I think you have a better sense of what's left on your pantry shelves, and if those high summer ideas for meal plans are holding up in reality. Are you craving green vegetables on your plate, or are you happy with the starchy, rooty, carb-laden comfort foods of winter? Is your tomato shelf surprisingly full, or are you already skimping on opening jars of salsa?
So, back to the numbers game: how many seeds to start? My rule of thumb is 50-100% more than you want. For example, if I want 2 tomato plants of a specific variety, I start 3 or 4. That allows for a dud germination, plus a spare plant in case of calamity (death by frost/hail, or a being stepped on by a dog). I usually end up with a flat's worth of spare plants at the end of the planting season, but there are always friends/neighbors to share with, or a food pantry or community garden that would love to take them. It also lets you select the best/healthiest specimens to plant. If I have a notoriously poor germinator (for me, parsley) or older seeds, I will double-up on the seeding to make sure I have enough starts, you can always thin out your pots later.
And, just because it's freezing cold and snowy out, one last summertime photo to remind me what this is all about: