If your niche has no relation with each other as you stated then Yes one Niche per domain.

However, as you just recently started you would be better off to just focus on the Niche you are most passionate about and one you feel will attach your readers to your site.

Doing too much at the start will likely create more stress and headaches one that could make you feel this is too much.

Once you gain experience and have already built up your first website and are comfortable where you are then and only then think about creating another niche website.

The best start is the training to get a greater understanding and as you build a solid foundation which Kyle will lead your through you will gain more confidence and skills along the way.

Wishing you well

Andre

Yes, that would be the best way, with keywords in your domain name. But, focus on one niche at a time.