I’m new to indoor gardening in Montreal. I set up a plant light with T5 bulbs but I’m still confused about a few things. The seeds I’ve started so far are: arnica, valerian, yarrow, patchouli, wormwood, marshmallow, queen anne’s lace, evening primrose, elecampane, marigold, lady’s mantle, and wood betony.
How many hours a day should I keep the plant lights on? Do I need to water both the bottom and the top? Also, I didn't know I should start with only 2-3 seeds and sprinkled a bunch inside each little compostable cow pot. Hoping I still have a chance with them.
Oh, dear! Except for the marigolds, you have chosen some very difficult subjects! While all of those herbs DO make seeds, they also propagate themselves vegetatively, by growing new plants from well-nourished root buds. A root bud with roots attached is a much more advanced organ than a germinating seed, so you will always get a bigger, better plant by choosing a start, or rooted cutting, rather than seed when starting hardy perennial herbs.
Annual herbs including basil and dill are much easier to grow from seed, because this is the natural way for annuals. Their life cycle calls for them to sprout, grow, and develop mature seeds for the next generation, all in one growing season.
Perennials have a different agenda: Survive the first year, then develop a root strategy that helps them live forever. For perennials, seeds are often an afterthought.
Not trying to get personal, but from the looks of your plant list, I think you would really enjoy working with scented geraniums as year round indoor plants. Here's a Canadian source: https://www.richters.com/Web_store/web_store.cgi?page=SubIndexPages/Geranium.html&cart_id=111.100
Scented geraniums are wonderful plants to grow under lights through winter, and you can put them out on a patio or deck in summer. Swish your hand through the leaves, and the fragrance will repel mosquitoes, at least for a while. Anytime you like, you pinch a leaf and breathe in the aromas. Some scented geraniums can be dried for their fragrance, and all bloom when they are in the mood.
As for your cultural questions, think perfect weather. Simulate rain from the top twice a week, and don’t worry about bottom watering until the plants have roots. Twelve to fourteen hours a day of light is good, more than eighteen is too much. If you have too many seedlings, use tweezers to pull out all but two of them from each pot. The germination rates for herbs and wildflowers is often quite low, but you may get lots of marigolds!