In case if concrete box girder bridges how is the number of cells determined?

It depends mainly on the depth of concrete box girder related to the width of the bridge, if the box girder depth > 1/6 bridge width so it shall be designed as single cell and vise versa in case it's less than 1/6 of the bridge width. 

Athough this not in my specialist and after i read: 

But the princible to carry the wieght load from a point inidefference as to whether the pending moment in positive and negative forc to there torsional stafness from the point of view of economy.

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