Well I generally look to make around 5-10%, I imagine the average odds ratio (ie back odds to lay odds) is probably around that. You don't make a huge deal in one go off of one win, but each 5% adds up.
I match bets on everything, basic 'arbing' and for bonuses, the only difference really between an arb on a regular stake vs on a free bet stake is the calculation you do.
The hardest thing I'm finding right now is trying to not get limited or closed down by the bookies - I'd love nothing more than to never win at the bookies so all my money flows over to the exchanges, but of these horsies have a habit of winning now and then!
As for how many books there are, have a look at the TGT bonus pages that will give you an idea. At a rough estimate on new bookies per week, maybe one or two?
And just how do they figure you out? Are you making some obvious mistakes, like stakes with decimals? Or something else?
How do they figure me out? I keep winning too much that's how! Unfortunately bookmaker's aren't charities or ATM machines and when you withdraw a lot more than you deposit funnily enough they don't like it!
There's no way around it, if there is I'd love to know but there's no practical way to NOT win ever at a bookies (otherwise what's the point in backing anything, you might as well just lay everything and retire to barbados next month heh). The myth that high odds is a good way to avoid winning is just that, a myth, eventually you get a 25/1 shot come in and you're scoobied, you've just drained your exchange account for a few grand and got a massive black mark against you at the bookie if you try and withdraw (well, I don't know for sure but if I were a bookie I wouldn't take kindly to someone taking a few grand off me after only giving me £50-100!).
There are probably another 2 or 3 million criteria that bookies use to spot bonus abusers, arbers, etc as well, there's just no way to know what exactly those criteria are. They could use sophisticated software to spot patterns, they might just have one guy that goes through and kills any account where the person's withdrawn way more than they've deposited.
The kind of thing you mention like using decimals (punting 13.23@101!) is probably one of the criteria at some of the bookies but who knows... probably not even the majority of the staff at a a given bookmakers actually know what criteria are used to lock down on people let alone the general public.