Do scope take up a slot after being combined with a gun?
No, if the scope is combined with the gun, it counts as one card.
You can see this in the rules:


  Specific  Equipment  cards  can  be  combined  to  build  im-proved 
  weaponry.  Take  the  two  appropriate  cards,  which  must  be 
  already  stored  in  the  Survivor’s  inventory,  and  combine them
  into the new weapon. This does not cost an Action, and the new weapon
  takes only a single slot in your Survivor’s inventory. When built,
  Survivors may immediately and freely equip improved weapons to their
  hands.
  
  Sniper Rifle: Combine a Rifle and a Scope to get a sniper rifle. Put
  the Scope card under the Rifle card. Now you can choose your targets
  when firing with this Rifle