In the homework section 2 of lesson 11, problem 5 askes how many clocks. I assumed since they are round, I would use なんこ for the counter. But in the answer section it uses いくつ as the counter. Why? Would it be understandable to a native speaker if I used the wrong counter?
As for とけい (clocks), it is considered a general object. Therefore, you can use both なんこ and いくつ counters.
The object being counted does not have to be perfectly round.
The いくつ counter is the most common and versatile counter. When you are not sure which counter to use, you can use the いくつ counter. Even if it is the wrong counter, Japanese people would understand. It would be good to ask them directly :)