According to industry insider shinobi602 on NeoGAF, Mass Effect Andromeda will have a very large number of planets with several activities. Shinobi602 stated..
“There are a certain number of very large planets that have any number of activities on them. Enemy bases, main plot missions, and other things. These are sort of the ‘open world’ areas you’ll freely explore. Some will be more story-centric than others. There’s also a lot of side areas too that could be smaller planets, outposts, derelict space stations, etc.”