If I don't buy the DLCs will I still see the planes as opponents in career mode ? Or do I need the DLCs?
I was thinking about getting 6 or 7 DLCs but want to understand how that works .

No, all the planes are as AI-plane included. So let's say you only have only the Albatros DII, you need to start in career mode an jasta that does have the Albatros DII, but you can still encounter AI-planes like the Fokkers, Re8, Fee or whatever is in the game. Only you have to buy these planes if you want to fly it.