How much RAM is really needed in an MSX 2 ?

256kB will do for more than 95% of the software, I guess.
512kB used to be nice when you still used RAM disks, printer buffers or for stuff like FastCopy using only one disk drive.
And maybe for SymbOS more RAM is nice

I would say: 1024 Ko RAM (512Ko strict minimum to feel at ease)

- commercial disks : 128Ko max
- commercial ROM* images : 512Ko max (*of course loaded from disks or tapes only by masochists who own the original ROMs, for obvious copyright reasons... )
- Symbos & generally speaking possibility to use 720Ko RAM disks : 1024 Ko
- Demos are more likely to use as much as they can.

Technically speaking also: it's quite easy to build a 1Mo Simm RAM into an MSX2, like Philips machines. 2Mo would have been just about the same, so arguably a good choice too. I've soldered one myself in mid-1990's (NMS8250). Without being an expert, it was OK. I had to fix a couple of solders since then, I also note that totally emptying the RAM sometimes requires to turn off the MSX2 for 30 to 60 seconds.

The more the better, but I doubt the fact over 1Mo RAM will really be of frequent/any use.
Multitasking makes big RAM sensefull anyway. Symbos reaches itself the 1Mo limit, and with it the hope of ever taking advantage of the full 4Mo mapper offered by the MSX2 system kind of disappears. Don't get me wrong: 1 Mo in Symbos appears to be a lot, more than enough for 3.57Mhz Z80 (the number of simultaneously running applications should be limited for speed reasons)! Moreover Prodatron tends to think 'Buffering' (mp3 loading, videos etc.) would be no less efficient from flashdisks than from RAM.
So, IMHO, 1Mo should definitely be enough.

Now it's a question of programmer's choices and needs.
By the way, what ARE the software/demos that ACTUALLY use over 1Mo RAM memory mapper, if any? (even optionally) Does anyone know?

Sin

I would say: 1024 Ko RAM (512Ko strict minimum to feel at ease)
By the way, what ARE the software/demos that ACTUALLY use over 1Mo RAM memory mapper, if any? (even optionally) Does anyone know?

I'd say: anything related to video. Things like Sphere/Wings are good candidates (Sphere/Wings uses ~ 350kB worth o' frames, and that's about 1000 of 'em then. The whole demo including tune, player, code etc. fits 512kB. With 1MB there's space for another 512kb worth o' frames. Assuming a single frame is about 350 bytes average then that's over 1400 frames. Or simply said: a 80 seconds linear movie at 30fps.

Normal/classic demo effects are usually very small, so for anything traditional, all that RAM is prolly not really required.

I do see another application for game intros, 1MB is truly a lot to hold bitbusted screen5 images! But as usual, technically it sounds nice, but I wouldn't know anyone around here able to design all that. We lack artists.. ._.

Not that I know of, apart from Meridian that requires -iirc- 512kB for its system/GUI and uses quite some RAM for its patterndata. But as it's not released yet as finished product it's a hypothetical app for the moment.

Orcacle can use it But most of these programs are just capable of using it and do not require it.