If you are using in the house 17 watts is plenty for one flat. Some have a thermostat built in, or a heat setting switch. If you place them on a cold floor, like tile or concrete it will draw some of the heat away from the mat.
I have a hydrofarm 17 watt (no thermostat) that does fine. you will still want a thermostat to place in the soil to make sure you are not getting it too hot. ( mine is designed to raise temp 10 to 20 degrees over ambient).
I made one out of small Xmas tree bulbs that pull 19 watts, I have it insulated with foil backed fiberglass. when fastened in a plastic bag I can get well over 100 degrees, which is too hot. I have it hooked through a light dimmer switch to control heat.

Larry

correction, I should have said "THERMOMETER" in the soil

I don't even know that you need a heat mat if you are starting seeds indoors in a heated space. Obviously if you were starting them in an unheated garage or basement or something, a heat mat would be helpful.

When I first bought my heat mat about 10 or 15 years ago, I thought I'd use it all the time. However, I found most seeds started on the heat mat germinated only a few days earlier than seeds germinated without the heat mat, leading me to question if I needed it at all.

This year, I germinated, so far, all my cool-season seedlings without the mat and didn't even use it for the tomatoes, peppers and warm-season herbs.

You really don't need a heat mat...it it just something that is marginally useful when starting plants from seed in an already-warm room. Even when I start tropical stuff or real heat-lovers, I can just place the flat on top of my deep freeze or refrigerator overnight for the first day or two. Then, as soon as tiny green shoots appear, I move the flat to the light shelf if it is cold outdoors, or straight outside to the patio or greenhouse if the weather is warmish.

For raising cuttings, a heat mat probably would be considered more helpful or more essential.

I think 17 watts would be plenty indoors, and without a thermostat you can expect it to raise the soil temp in the flat 10-20 degrees indoors. I never bothered buying a thermostat since I don't use the heat mat much anyway. The longer I garden, the more I learn that we don't really "need" a lot of the stuff that gardening magazines and garden catalogs try to convince us we must have. These days, I am into doing whatever is the simplest and can be achieved with the least amount of fuss.

Dawn