Combine them until you have the same amount.
Next combine textures into an image.
Next drag the texture to 3DS Max.
Oh NO! It looks kinda broken. Well the reason for this is because the UVs stay in the same place so we need to edit them.
Goto the modifiers tab and add a UV Unwrap modifier like this:
Now open the UV Editor.
We need to fit these UVs so they are with the texture but also in the box. (It has to be in the box or else you will get an error)
Right click and use the "Freeform Gizmo". This tool lets us scale and move the UVs in a very controllable manner.
Nice we got it! Though separate textures are still not working?
Lets fix this! Select the white box and select the model you separated. It will then highlight the UVs
Now it's done!