how many parts in whole a2,m4??

You could start counting.

And before long you'll have to decide to what degree you want to break it down. A mag catch is two pieces and a trigger guard is four, but most of us get those as one piece.

A quick count gets me 22 pieces for a barrel assembly:

barrel  
barrel extension  
barrel indexing pin  
flash hider  
crush washer  
FSB  
FSP  
FSP detent  
FSP detent spring  
FSB taper pin  
FSB taper pin  
sling swivel  
sling swivel rivet  
barrel nut  
delta ring  
weld spring  
snap ring  
handguard half  
handguard half  
handguard endcap  
gas tube  
gas tube roll pin

Somebody will probably be along to tell me what I forgot. Anyway, that's all the fun I'm going to have with it.

ETA - Above is for the A2 and doesn't count the pieces included in a side sling swivel mounting.
EATA - And I forgot to break down the handguards and heat shields - I'm done.

i just thought someone did this exercise at some points every other one has

I saw a tee vee show a while back that had a complete A2 laid out with its various parts counted at 127 or 129 pieces. I don't remember which count is correct.