I have building with 16 showers , 6 toilets, 8 sinks and 6 urinals what size of water pipe should be for the supply?
In the U.S., water supply requirements are calculated using a table of water supply fixture units (wsfu) in the plumbing codes. Below, I use the values for a public/commercial installation (residential requirements are less but the OP's question appears to be a commercial setting):
|6 toilets - flush valve||60||0||60|
|6 urinals - 1" flush valve||60||0||60|
Notice the total values are not a sum of hot + cold; they are also from the WSFU table.
After determining all your WSFUs from all types of fixtures in the group, you need to know the minimum water pressure and maximum supply line length. These values, and the required WSFUs, allow you to consult a second table giving you the minimum supply pipe & meter size. In this case, a 2" supply should suffice.
This calculation should be performed for each branch of the plumbing system.
You should not invent your own method based on pipe diameter because the tables in the building codes already account for pressure drop due to length of the supply lines, and available supply pressure.