As a new player with a small collection it has become obvious that some cards are much more in demand than others when it comes to deck building. Nearly every deck with 8s or 10s seems to play multiple copies of Steven wiles and unprepared respectively. Are there any cards which are worth having more than 4 of, assuming you’ll be having 4+ decks built at any given time?
I think with 4+ decks built you’d probably be needing more copies of your favourite Cheatin’ Resolution actions. As for the rest, it’d depend on your play style and values you were using as to what you’d want more of, but as 8s are good for a lot of decks you might want more copies of the deeds at the very least, and at least one Steven Wiles is commonly splashed into lots of decks whether they run 8s or not.
I maintain 5+ decks at once, I try and keep one competitive and the rest are more for tooling around and/or high concept decks. Buying one of each expansion should be good for you, with the notable exception of jokers, but anything can be put in for those if you use card sleeves.
Ok, so cheating resolutions, deeds and Steven wiles…
Buying one of each what?
I presume 1 of each pine box/saddlebag (i.e. the expansions, you need 2 copies of the base game to get 4 copies of each card).
I don’t find really find the need for more than 4 of any card. The promos from the prize kits are generally good at providing more used cards and also a lot of the errata cards are useful (Coachwhip and Unprepared). Kidnappin’ is the only card I probably want more of but even so, when I do I just use other cards of the same value/cost and then put in a little bit of paper (I sleeve all my decks) that I don’t really feel the need to buy more sets just to have more. YMMV.
If I were to buy singles I would look for the cheatin’ resolutions on 8 and Q - This’ll Hurt and INWYK.
Circle M Ranch, Unprepared and Steven Wiles are usually the cards I play in most of my decks.
The thing with Deeds is you generally only need 1 of them in any deck.
I’d kill for some more Fancy New Hats 
I find one of expasion serves me well, the sheriff kit helped with more jokers and the other kits have been pretty good about the cards included. It also depends on the decks you try to keep together at the same time. If you build everything around the same values and structures you tend to need more copies of the same cards.
How cool would promo Fancy New Hats be with hats from prominent characters. I would love a ‘Ghost’ hat.