How many loading cycles are possible by the iPhone battery? On the one hand there are batteries with around 1000 loading cycles and 90% capacity, on the other hand you have batteries with 500 loading cycles and 80% capacity.
Between 500-1000. If you have a problem before warranty is out then take it to Apple. If your phone is out of warranty, replace the battery. Pretty simple. It's a piece of telephonic equipment, not a space rocket.
There isn't a set number of cycles, that depends on the condition of the battery throughout it's life. Having the phone in extreme cold and hot conditions shortens it's life, charging it using the iPad charger effects it (the slower the charge, the better for longevity), how hot the phone is when it's being charged, if you use it while charging, etc.