Sis actually. 

1. What is the "MCB C20 WH"? is it different type of other MCB?
Standard C20 MCB.
WH denotes the one for WH circuit.

2. if I have 4 header, which mean I will need FOUR 10mA RCD?
Yes correct. WH 10mA RCD is per unit of WH. Each WH should be on its own dedicated circuit as one WH enough to use 15-17A nominal (WH is heavy appliance). Your 2.5mm cable only support 21A. And the 10mA RCD, only come (from branded mfg) up to 25A max current. In short, the standard cable sizing, the RCD current capacity (max 25A) all only suited for single unit WH as it is sensitive device (0.01A /10mA).
Whilst there are champions from china making 10mA RCD that can handle 40A I also dare not use.

3. The purpose of 10mA RCD is to trip when the heater leaking to our body? my current installation if leaking it won't trip?
Your current installation only trips at 300mA - by then, the human dieded 6 times over.
As the mains is 300mA, it allow 300mA leakage before it trip.
A human RIP at 50mA.
[attachmentid=10803044]

4. In short, I only need to have 10mA RCD in between the MCB and Header individually? Any other wiring need to be change/improve?
Yes, correct.
RCD is comparing input and output current difference. Input and output must = equal the same +/- RCD tolerance of say 10mA (0.01A).

Meaning, if WH input is 16A on L, at least 15.99A must return on WH output (N) else it has deemed to be lost somewhere.

Lost somewhere mean could be leaking through human.
So when the return not enough (<15.99) say only 11A return. The 4.99A cannot be lost, it mean it leak. As 4.99A far exceed the 0.01A allowed by 10mA RCD it will immediately trip.

MCB only consider single wire (L) amount of current passing through (short circuit prevention).
For a C20, meaning the speed limit is 20A.
Slight overload at 23A will trip the SLOW thermal overload after say 20 min.
Major overload at 100A will immediately trip the magnetic overload.

5. between I'm quite confusing are ELCB & RCCB the same? can you correct me if the naming in the image below wrong?
The difference is how and where they measure.
ELCB only detect earth current rise >0 leak (L to PE) leak on that appliance's earth. If it leak via anywhere else (L-N, or L-other PE eg wet floor, L-human) it will not bother. Hence in short I just say it protect that appliance only.

RCD because it constantly compare L input =N output + allowed tolerance (like 10.00A in on L, >9.99A must return as allowed tolerance is 0.01A for normal operation things like wire resistance that do NOT constitute a fault condition). It does not care where it leak through, RCD always compare in and out current so if it leak through any method (L-N, L-PE, L-other PE, L-human) it always trip.

Note: There is something called RCBO which do both function of MCB+RCD in one.
6. Appreciate if you can share the image of "MCB C20 WH" and 10mA RCD
MCB C20 is any usual C20 (20A MCB). WH is just designation for the WH circuit.

10mA RCD (must be used in conjunction together with C20 MCB) ie. add this along with MCB C20
ABB= https://www.lazada.com.my/products/abb-f202...i2IABn&search=1
Hager= https://www.lazada.com.my/products/hager-cc...l5n2dU&search=1
Schneider = https://www.lazada.com.my/products/schneide...8t5PsK&search=1

10mA RCBO = RCD+MCB in one package pao kah liao 2 function in one
Schneider RCBO https://www.lazada.com.my/products/schneide...PVCaV4&search=1
The drawback is, officially, their MCB is a C25 equivalent so you need to use with 4mm cable. As 2.5mm cable only good up to 21A rating.


First one from left  is MCB 3P (3 pole) C63 (63A)
Second one is 4P RCD but this is 300mA. Only used in commercial/industrial where they use heavy motors. Not safe for residential and its against regulation, as the human has dieded 6x before this trips. (RCD/RCCB/RCCD = same function with diff name like zebra crossing=pedestrian crossing)
Third one is MCB 1P C10 (10A)

You will then ask, why do people still put the 300mA in residential?
Just to save the few bucks for more profit/lower tender cost

For 4P, 300mA cheap ma coz insensitive.
They sell the 300mA because still valid use for industry.
Not all RCD created equal

Should use the 30mA then up to par with current residential standard. 4P 30mA 63A usually cost almost double of the 4P 300ma.

Personally I'd still try to swap out to 30mA, safety first.