Can't find CD4050 library in Proteus 8?
I haven't worked with Proteus but most electronic CAD packages have similar behavior when dealing with individual gates. 

That is: each gate is individual. You place all of the individual gates in your schematic as required. Then you gather the gates into the desired packages. 

My CAD package names this process as "packaging". You do this in the stage between completing the schematic diagram and starting to PCB layout. 

Packaging assigns the individual gates to the physical chip. 

The information about this process should be in the user manual for your particular CAD program.