The "featurephone" OS is S40 and it's in both "classic" phones (e.g. 515) and Ashas.... unless there is something I've missed completely. And come to think of it, I haven't heard any confirmation of closing the production of S40 phones, so I guess that'd make it 3.
The OS in the most basic of Nokia phones is commonly referred to as S30. It doesn't really need much active maintenance, though, because they mostly only change graphics and included games (and utilities) between those phones.
The classic feature phones is an older Symbian variant and Asha Platform OS is S40 from what I can tell. Symbian is still king in a lot of countries so Nokia will have to support it for a long time.
Struggle to see how this helps Nokia. Nokia Xs are smart phones at a price of a feature phone. Great! Welcome to an already very crowded low-margin market of cheap Androids in India, Brazil, China and Africa, the same market MS will enter with cheap 8.1 devices and where Firefox OS devices are heading. Yet, Nokia Xs have little extra to offer to the consumers that would significantly differentiate themselves from the competition.
Asha did well in developing markets, and Nokia has an attractive app/ecosystem suite for low-end phones. But Series 40 OS is not competitive with Android. Nokia X goes straight at that problem. If you are careful about the apps, especially the browser, Android will run well on VERY cheap hardware.
Symbian isn't supported any more even today and Ovi Store has been frozen. And Symbian is much more of a "smart" phone OS than WP8. Also S40 isn't really Symbian. :)
