Central whereabouts? Are you interested in West, North or East (just going by St Paul's and City)?
Channing, South Hampstead, Highgate and Forest are the North/East ones (latter two co-ed).
The London Consortium includes Channing, South Hampstead, Godolphin, Notting Hill, Francis Holland x 2 and others. They've introduced a new paper that you can do in any of the schools you apply for though I wouldn't apply to loads for the hell of it as you'll still have to do interviews and open days etc (and don't underestimate how laborious this is).
For our two we limited ourselves to three exams (though for the girl, that was four schools as two were in the consortium). I honestly think that's loads so long as you're sensible. It seemed to take up the whole of January as it was (3 x exams, 4 x interviews, one of which was scholarship so took all day etc, etc).
If you were to apply to St P and City, I'd add a slightly less academic school and then a banker. If you look at the Sunday Times list that should give you an idea (e.g. v academic but not St P would be LEH or G&L, banker would be FH). However, it's not straightforward - I've known girls who have got into City but not one of these and vice versa. There is a lottery element.
Also make sure you visit. We didn't like City as much as schools that she was more likely to get into so we didn't apply to it - there's no point in applying to one that's a long shot that you don't even want that much, especially as you can end up feeling obliged to take it for the prestige.