For anything but the Core set, you will have enough cards from buying two copies of that product. It is worth noting that this is because these set only provide 2 of Lengendary or Unique cards. Yes you legally can have 4 copies of the same Legendary card in your spellbook but there really is little reason to do so. Still though, if you want to have that option you'll need 2 of each.
For the Core set itself, some of its cards only have one copy, and there is also only one copy of that card in the spell tome that contains it. So you are looking at 1 core set + 5 spell tome 1's + 5 spell tome 2's at cheapest.
