Bum marketing questions

Yea, its nice to have your landing page ranked, but chances are the competition is a bit too stiff. When your starting out its far more important that your landing page converts then being keyword rich. You wont have the backlinks to get your page ranked initially.
Once you're ranked you might want to consider adding some SEO content to your landing pages, but when you start out having a quality sales page is way more important.

Thanks, but what about the domains? One for each keyword pointing to a one landingpage or several landingpages?

You don't even need to have your own domain for your landing page. Just use a free blog or lens or what ever. It all depends on what you want to do.