You can submit one Hub to be reviewed every two weeks for a Niche Site. That does not mean per Niche site, it means for all. To my knowledge, HP is still vetting existing Hubs deemed acceptable by some criteria but may look at submissions first. That is because the editor staff is about a dozen (?). I think, though unsure, the criteria is germane to a Niche Site, quality, and traffic history.
Footnote: There are many Hubbers on the Forums with experience and expertise with the Niche sites and submissions. It is not uncommon for discussions to be there about quality and how-to stuff about submissions. If you wish to post a question there on that topic I suggest going to Forums default page and then posting under HubPages Community → Getting Help from HubPages Community.
I have had two Hubs moved, but they were chosen by HP. I have not tried submitting one yet. Yes, the earning potential is much greater at Niche site. That is because of how Google views websites today. Specialist sites are valued more.