I also often buy whole fish fillets - either fresh or frozen (and occasionally breaded). I always assume that there will be bones, so if possible rinse the fillets and look / feel for bones prior to cooking. That doesn't deal with ones that are in the body of the fillet, so I take care to chew slowly and feel for bones as I eat.
I have found bones in every type of whole fillet, including breaded ones, but not that often or that many. I have returned the remainder of a package of pickerel fillets that averaged out at more than 20 bones per fillet for the first two that I pulled out, but most often there are no more than 1 to 4 in a fillet.
I've been filleting my own fish since I was a wee kid (I was brought up with "you catch it, you clean it") and know that I have to take my time and accept that there will be some "wasted" meat sometimes to make sure all bones are out. Industrial processors can't take that time and need to minimize waste, so you can be sure that there will often be a couple of bones missed.
I've never purchased fish from a market or in a package that didn't have a warning label about possible bones slapped on to it - and it is there for a reason!
Sir Terry Pratchett: "Science is not about building a body of known 'facts'. It is a method for asking awkward questions and subjecting them to a reality-check, thus avoiding the human tendency to believe whatever makes us feel good."
It might depend on the type of fish you are buying, or perhaps the brand, as in the fish aren't filleted the usual way of either side of the back bone. I wouldn't be worried, so long as you are careful eating it.
I remember as a child most of the fish my family ate had bones in them and we were always warned to watch out for them. It's only in later years that the boneless method seemed to have been adopted by most suppliers.
BLAST - NOW I want some (crispy) Battered Fish and Chips with a little vinegar on them. Haven't had them for a about 3 years, but it is bed-time LOL!