As davekeiller said, the West Highland Way is quite popular and you're likely to be within sight of other walkers for the duration, especially if you do it in the summer. That being said, it's popular for a reason - it's a very fun trail!
If you do almost any other Scottish long-distance trail, there will almost certainly be fewer people. I did a section of the Hebridean Way in June this year and met an average of 2 or 3 other walkers per day. On the Rob Roy Way and Speyside Way, maybe 5 or 6 other walkers per day. I haven't done the Cape Wrath Trail, but I know of people who've gone days at a time without seeing another soul on it. If solitude is what you're after, a few long-distance trails I'd recommend researching are the Cape Wrath Trail, the Affric Kintail Way, and the East Highland Way, all of which are more remote trails and therefore less heavily trafficked.
I don't camp (yet!), but my understanding is it usually isn't too difficult to find isolated wild camping spots. But if you're planning to use or camp near bothies you will probably have company, especially in summer.