How much RAM is really needed in an MSX 2 ?

Depends on what kind of apps you're using. Most MSX2 stuff works fine with 128kB RAM/128 kB VRAM.
For example, I use the rom of my Philips NMS 8250 with openMSX, that's my base configuration.

That's the point. As your posts suggests, the only apps that really require great amounts of RAM are those created lately. The software made in the golden age of MSX works well with just 256 Kb or even 128 Kb. I was in doubt between buying a 4 Mb ram expansion or the megaflashrom SCC of Pazos. I choose the megaflashrom just because I thought that most of the software that requires more than 128 Kb are the megarom to disk conversions. The megaflashrom SCC solves two problems: the need of RAM and the need of an SCC cart which I didn't have.

...but I'm still pondering about buying that ram expansion !

256KB (inner and faster access) Memory Mapped RAM , 512 KB megaRAM and 3 1/2 720 KB should be fine for most applications or games.

With 512KB RAM it is possible to run things like the demonstration programs that the MSX community released in the past years.

Matsushita Video Graphics probably would require 512KB Memory Mapper.
Philips Video Graphics works with only 64KB RAM, but the animations would require extra Mapped RAM to work.

512KB Memory Mapper and 1024 KB megaRAM would be great! Maybe later, there could be software for 2048 KB memory ...

Graphics or animations would need 256KB or even 512 KB Memory Mapper, for general games and demonstration software even a 512KB megaRAM would do fine!

I think that most Japanese cartridges only need to use one page of RAM (8/16KB).

MSX1

First 16KB (probably ROM BIOS)
Second and third 16KB pages = game ROM/EPROM
The Last page with 16KB RAM is what the program need .

16+16*2+16=64KB memory

Sure, it is possible to use external 64KB RAM for other purposes. Since a lot of software do not make correct use of bank switch, there is not any need for more than just 8/16KB RAM (the Z80 only see 64KB of memory).
When a personal computer only have 8KB RAM, the memory is rumored to be mirrored into the upper part of the page.

Since the MSX2 has full 64KB RAM and probably more than 64 KB VRAM, it would be more than enough for most cartridges ...

I'd say in the world of MSX, about 512K is enough for everything! (640K pun intended)

Here is the rationale:

1) The holy grail of all cracked games, translated Metal Gear 2, will work on 256K, but with lots of swaps and no SCC support. On 512K you get fewer swaps and some of the best SCC music (SCC cart with disconnected ROM required in MSX slot).

2) 512K gives a decent size ram disk, if you use Turbo Pascal 3 or something like this.

3) More then 512K is kind of pointless due to general slowness of MSX.

4) Final MSX computer had 512K from factory (TurboR GT).

5) You can upgrade all Panasonic and Sanyo MSX2+ computers from 64K to 512K simply by adding 4 x 44256 DRAM chips. Also it's teoretically possible to add 512K RAM to all S1985-based MSX2 computers.