You should be able to get 10-15 corny kegs out of a full 5lb CO2 bottle. Now this is considering that everything is ideal in your setup. (your system is airtight with no leakage, minimal headspace when carbing kegs, etc.) I have yet to get this number of kegs out of one 5 gal bottle, but more like 10 due to purging prior to filling my kegs or minimal leakage I suspect.
I've lost count, but I never get 10. I'm on my fourth bottle and I'm nowhere near 40 batches since I bought my setup, I don't think I've even done 20. 1 bottle leaked empty on the 2nd batch, one on the 5th or maybe it was just done. The third lasted a while but I wasn't counting. I'm on my 3rd batch on the 4th bottle at this point.
I keep 2 tanks of propane but still manage to run out when I've got prime porterhouse on the grill...
Leaks suck. I have a spray bottle of starsan that I use to check connections. My co2 tanks usually last at least 5 with force carbing and cleaning my lines. Leaks have led to one keg for a 5# tank on two occasions. Those were expensive screwed ups.