Too many.
I experimented for years with rucksacks until I had an epiphany. I've all sorts of bombproof, 1000 denier jobs with elaborate suspension systems, as well as traditional canvas and leather, until I realised that that up to a quarter of the weight I was carrying - depending on the load - was the rucksack itself! I've started walking more than fixed camping and weight has become an issue.
I finally went with a ULA Circuit pack which comes in at 41oz - which you can lighten further by removing some of the removables, like the hydration bladder sleeve, for example.
It's not the lightest pack around, but it has an excellent strength/ robustness to weight ratio. The size is perfect (around 60 litres with a 45 litre main compartment - expandable due to its roll top closure). This enables me to keep my summer pack weight down to around 20-25lbs and winter, a hair over 30lbs.