20mm = .787 inches
15mm = .591 inches
5mm = .197 inches

A simple program for performing a variety of conversions can be found here:
http://wscope.com/convert.htm

Before I begin I strongly suggest you get professional help or education. This fundamental lack of knowledge will be expensive for you and your company and might get someone killed.

There are many people here that will help you set up your program for a small fee.

You can sign up for a class in Houston, TX on February 22-24 here:

http://catalog.asme.org/Education/ShortCourse/BPV_Code_Section_IX_Welding.cfm?ProgramNumber=SC3607&CFNoCache=TRUE

The cost is about $1600 American, figure on an additional $1000 for the hotel and at least $200 for meals while you are there. WORTH EVERY DOLLAR

There is a program developed by one of the members of the Section IX committee that takes a lot of the guesswork out of WPQ/WPS writing:
http://www.cspec.com/

Welder A+Welder B qualified range?  QW-452.1 (b) Make sure you look at notes 1-3

Welder A qualified range? QW-452.1 (b) Make sure you look at notes 1-3

Welder B qualified range? QW-452.1 (b) Make sure you look at notes 1-3

If we have 50mm plate to weld, can we use this welder A deposit 20mm+welder B deposit 10mm+welder A deposit 20mm? -  Yes, QW-452.1 (b) welder A is qualified for unlimited as long as there are more than 3 layers of weld metal, you do not need welder B.

Maximum to be welded(means:weld metal thickness is Unlimited?) - The welder is limited to the maximum listed on your weld procedure (WPS)

Again,
I strongly suggest you get professional help or education. This fundamental lack of knowledge will be expensive for you and your company and might get someone killed.

There are many people here that will help you set up your program for a small fee.

You can sign up for a class in Houston, TX on February 22-24 here:

http://catalog.asme.org/Education/ShortCourse/BPV_Code_Section_IX_Welding.cfm?ProgramNumber=SC3607&CFNoCache=TRUE

The cost is about $1600 American, figure on an additional $1000 for the hotel and at least $200 for meals while you are there. WORTH EVERY DOLLAR

There is a program developed by one of the members of the Section IX committee that takes a lot of the guesswork out of WPQ/WPS writing:
http://www.cspec.com/

Welder A is qualified for a maximum deposited weld metal thickness of 30 mm if the qualification weld was not made with at least 3 layers.

Welder A is qualified for unlimited deposited weld metal thickness if the qualification weld was made with at least 3 layers.

Welder B is qualified for a maximum deposited weld metal thickness of 10 mm.

"If we have 50mm plate to weld, can we use this welder A deposite 20mm+welder B deposite 10mm+welder A deposite 20mm?"   No, this is not permitted if Welder A did not use at least 3 layers in the qualification test.  It is permitted as mentioned above if at least 3 layers were used.  There is an published ASME Code Interpretation that addresses this question.  I will find it tomorrow and post a copy.

"All" refers to all test coupon thicknesses, regardless of the number of layers.  If the welder deposits at least 1/2" (13 mm) of weld metal in the test coupon using at least 3 layers of weld metal for the welding process, then the welder is qualified for all deposited weld metal thicknesses in groove welds (meaning unlimited thickness).