Simple answer: if you're not a kit-lens-only shooter, five or six.
- Mid-range zoom (minimum 28-70mm equivalent, typically kit, though enthusiasts want something faster than kit lenses in this range)
- Telephoto zoom (minimum 70-200mm equivalent, typically the kit-lens partner, though enthusiasts want something faster than f/5.6 at the long end)
- Wide, fast prime (24mm equivalent or wider, f/2.8 or faster)
- Normal, fast prime (~50mm equivalent or wider, f/2 or faster)
- Moderate telephoto, fast prime (70-90mm equivalent, f/2 or faster)
- Macro lens (preferably 100mm equivalent or longer)
A kit with those lenses in it would let you make most of the general images you'd want. Wildlife, sports, and some event shooters would need more or different lenses, but you'd be surprised how much you can do with just that simple set.
So how do the mirrorless mounts fare?
- Canon EOS M: Only has a fast moderate wide prime and a mid-range kit lens. Can use EOS lenses via adapter to provide macro and perhaps some telephoto capability.
- Fujifilm X-Pro 1 and X-E1 (XF): Has the two of the three fast primes and the macro lens. Fujifilm in late 2012 filled in the wide prime and mid-range zoom in 2012, and their roadmap shows them filling in the telephoto zoom in 2013.
- Nikon 1 (CX): Has the three basic zooms (kit lenses) and a superzoom, but their first prime was not wide enough; their second prime is a normal lens that is near perfect (18.5mm f/1.8). The system is missing the macro, though you can use the 40mm or 60mm Micro-Nikkor on an FT1 to solve the problem, and solve it in spades. Still missing a fast telephoto, though it has been hinted at.
- m4/3: Has everything on the list, and many in multiple variants.
- Samsung NX: Has everything on the list, though some are in short supply as I write this. There lenses really don't go beyond this list, though.
- Sony NEX (E-mount): In late 2012 was shipping everything on the list, though the macro lens is short in focal length (45mm equivalent). Plus only two of the lenses really start to live up to the NEX-7's abilities in terms of resolution.
As might be expected, the "oldest" of the mirrorless systems fare better than the more recent ones, which is to be expected.
Beyond the basic lenses, things get more interesting. Technically, a well-rounded system would also have these options:
- Fisheye (180 degree diagonal capability)
- Wide-angle zoom (minimum 18-35mm equivalent)
- Full range of primes (24mm, 28mm, 35mm, 50mm, 85mm, 105mm equivalents)
- Fast mid-range and telephoto zooms (24-70mm, 70-200mm f/2.8 equivalents)
- Long telephoto zoom (300 or 400mm equivalent at long end)
And here's where we are today with that:
- Canon EOS M: two more lenses coming in 2013, but well behind the others
- Fujifilm X-Pro 1 and X-E1 (XF): wide angle zoom and additional primes coming in 2013/2014
- Nikon 1 (CX): one additional prime coming in 2013
- m4/3: has options in all of the categories; now adding lots of overlapping and specialty options
- Samsung NX: one additional prime in road map
- Sony NEX (E-mount): future roadmap a bit hazy, but now has serious third-party lens support
The net takeaway, at least here in early 2013, is that m4/3 was the first to a complete set of lens options (and with many multiple choices), and was already there in 2012. Sony would arguably come close to the basic set I outline, but still has some quality issues in a few of the offerings, particularly the wide prime. Samsung hasn't announced anything new in lenses or their road map recently, so while they have a decent lineup, it looks like they'll remain where they're at for awhile. Fujifilm and Nikon are in catch-up mode, and it will take them a fair amount of time to do so. It didn't happen in 2012, but both companies have pre-announced lenses that will help them fill more gaps in 2013. Canon is the laggard, with only two lenses available and only two known to be in development.
Of course lens size and lens quality and other factors come into play when deciding between systems. But just based upon availability, m4/3 is the clear leader in terms of serving their users' basic needs, and even manages to serve extended needs well.
Note: Obviously I love choice, as do most photographers. I'm not arguing that companies should only do the lenses in the two lists, above. I'm only suggesting that those are two good starting points to evaluate whether a system offers reasonable choice. One would hope that we eventually get to the level of the DSLR systems, where 30 or more lenses are available for a mount at any given time, and the used market increases those options as lenses come and go.