They said last month that there are more than a dozen "hubs", one of which includes the planet Kadara which had some planetary exploration gameplay shown already. And considering there are loyalty missions for all 6 of your squad-mates, I feel good about there being at least 6 planets that you should be able to "touch down" and explore on the mako. And I find it hard to believe you won't interact with the other species once they have touched ground on a planet. There may be some overlap on certain planets, but six being the minimum sounds right. I'm hoping for around 10 "explorable" planets.