is there any limit for the number of cpu,memory, hardisk, graphic cards on a single computer?( it might be weird to have a laptop to have 10 cpu )

there is no limit
 

a customer gets 10% discounts if he buy 2 intel cpu and 1 nvidia graphic card in a single computer, how about if he buy 3 or more intel cpu and 2 or more nvidia graphic cards on a single computer? did he still get the discount? (it makes sense if a customer get discount if he buy more, it might be weird he lose discount if he buy more)

as long as there are at least 2 intel cpus and 1 nvidia graphics card on the same computer then they get the discount.  the discount is only applied once even if they have more on the same computer.

if there are two computers with the required hardware (2 intel cpus and 1 nvidia graphics card then they would get 10% discount on each computer)
 

for the monitors, can I just make it eaiser to make only 2 sizes? (e.g 14" and 21")

since you only ask if there is a condition that require a monitor to have 20" or larger. (you know, small store don't have much selections)

the user can specify what size they want give them several choices some options below 20" and some options above 20"

 

about the dimension information: can we just make any dimensions we like? like laptop (14", 14" ,1") or 14 " monitor with 14" width and height but only 1" depth.

if its a laptop the dimensions can be based off of the screen size, if its a desktop then the dimensions are the dimensions of the tower (case size)

 

also I am kinda confused about the dimension of the desktop and workstation, does it mean its case size? or monitor's size? if it's case size, can i make it like (1",1",1")? (it might be a future technology)

see previous answer and no you can offer a few different case sizes that can be found today.

 

for the hardisks can I just pick only one manufacturers? so customer can only pick only one type of hardisks. and for the manufacturer's name need to be real? can i just make up a name like popo or dada?

you should look up a few real manufacturers and use those names.

 

does each single computer need every single parts? can a customer just buy a computer without hardisks, or graphic cards? (it might be weird, but you know there are crazy people out there)

a person can only buy a whole computer.

 

for memory card and hard disks, can I just make only one size? like hardisks has only 1TB and memory has only 1 GB. (I got a small store, so not able to have a lot of selections.)

you should give the user several options for hard drives and memory

for the very last part, you say do not prompt user if he want this special. does that means there is an option a customer can choose so his name will not appeared on the receipt? If so does this program should always ask customer's full name before taking orders? or just first, or last name? customer's name will always shown on the very end of the receipt?
you don't ask the user if they want the discount and you don't tell them about it, if they happen to choose the configuration that triggers the discount, then the discount is applied automatically.