not much
Not a lot (of interesting or exciting activity). A neutral, unenthusiastic response to someone asking what one is up to or has been doing. A: "Hey, Tom, it's been a while since I last saw you. What have you been up to?" B: "Oh, not much. Still in the same job." A: "What's happening, Jake?" B: "Not too much. Studying for my exams next week."
Not (too) much,
a response to greeting inquiries into what one has been doing. John: What have you been doing? Mary: Not much. Sue: Been keeping busy? What are you up to? Bob: Not too much. Sue: Yeah. Me too.