The maximum number of atoms present in one plane of an ethylene molecule is

Ethylene is a hydrocarbon containing double bonds with the formula ${\mathrm{C}}_2{\mathrm{H}}_4$. Ethylene has a structural formula ${\mathrm{CH}}_2={\mathrm{CH}}_2$. Here each carbon atom is ${\mathrm{sp}}^2$ hybridized. Thus, its shape is trigonal planar. 

So, all 6 atoms lie in one plane.