One of each is more than enough! I mostly own singles of kits and compete in tournaments all the time and have been doing well.
Personally I would actually recommend against getting multiples of any kit due to FFG's habit of releasing alternative paint jobs (e.g. Rebel Aces, Imperial Aces, Imperial Vets).
Of course, if you're looking to run a specific build or using multiples of ships with low cost (e.g. B-wings, TIE Fighters), then you might want to pick up multiple kits.
I don't own the A-wing, Y-wing, Decimator, TIE Bomber, K-wing, Tantive, Rebel Transport, Raider kits.
I have doubles of the Core Set (more TIE fighters!), Slave-1 (my favourite ship), IG-88 (best run in pairs), B-wing (best run in multiples), Phantom (Tactician crew card
), and Most Wanted kits (more Binayre Pirates!) only.