Nissan had around 20 platforms when the company was in financial strife... they then began whittling that number down to around 5. They may have added more since their turnaround.

It does seem like too many. I wonder how many platforms Toyota get by with considering similar sales numbers globally (forgetting for a moment they don't have too many RWD car platforms)?

 

Nissan has 3 to 4 total chassis.
There is whatever the Sentra is on, dont know about the new one
The Altima and Maxima are based off one chassis, just longer and wider. Murano is also based off this and I want ot say the Quest, but I dont think so.
Infinity has the FM chassis mostly. FM has 350Z, G35 coupe and sedan, M35 and 45, FX 35 and 45, and will underpin the new Skyline.
All the trucks are basicly based off the Alpha chassis, both full and midsized trucks.

GM does have a LOT of chassis. They need Delta or Delta II, something that can be more modern. Epsilon II needs to be flexable to offer a propper mid size car with good balance and handling. Then the Zeta cars to cover the low-cost RWD. Then there is Sigma II to cover all of Caddy, the 900's as well.
Kappa and Y body, they will always be a low production unique chassis.
IMO, there is heavy overlap in trucks. I dont understand how GM could not have made a pick up off the 360 chassis. Its a solid chassis, no doubt. That big 6cyl would have devoured Toyota and Nissan before they even thought about brining any mess into the midsize game. Could have had the 5cyl tuned for about 200hp and get better gas milage or juice up the 4cyl Atlas and have that as base, with the 4200 as top of the line. Would have made a killer mid size truck. And then there is always the ability to go V8. But the 4200 vs the Toyota 4.0 and Nissan 4.0, it would have been game over for the imports. I still, to this day do not understand why GM has 355. Probably the biggest waste of time.