This question is a bit broad and too subjective to answer effectively. Some people may think you only ever need one Force of Nature, other people might think you should have two. The same could be said about Brawl, Doomhammer etc. It's all pretty subjective.
If there was a particular deck you were wanting to work with, it'd be much easier to give advice as to which cards you could get away with only a single copy of. However, just in general? Hard to say.
In general, I think you can get away with a single copy of those cards for the most part in most kinds of decks you'd use them in. However, that's just my opinion, others may disagree. Anything lower quality should be easy enough to come by 2 copies of regardless and higher quality is legend which is restricted to 1 anyway.