I am having total 6 compressor in different cfm and my total cfm is 4755. what size pipe i can use total feet 300
Are they all connected to one main pressure vessel/tank? If so, connect all compressors with a pipe of right diameter (according to that compresssors output) to the vessel.
From the vessel to the factory, use the total cfm for the calculation. Check the pipe sizing section of this website: http://www.air-compressor-guide.com/compressed-air-piping.html