When to run one card instead of 2

I feel like your question is starting from an invalid premise. Hearthstone is no different from other card games when it comes to how many copies of a card you want in a deck (barring the obvious differences of how many cards you can have and how many copies are allowed).

In Magic: The Gathering, there are common strategies for putting anywhere from 1 to 4 copies of a card in your deck. I even looked at some recent Magic: The Gathering top decks and saw Akito Shinoda's deck which won the Grand Prix Shizuoka 2015 which ran a single copy of 17 different non-land cards. This is a top deck in a card game that allows four copies of a card.

In most card games, there are strategic reasons to use less than the maximum number of copies of each card.

So, onto Hearthstone! One thing to realize about many top Hearthstone decks is that they tend to run a variety of cards that draw cards. This means often when a player gets to the maximum mana (on their 10th turn), they have already drawn two thirds or more of their deck. This means that running multiple copies for reasons of consistency only matters for your first few turns. Here are a variety of reasons that you would only run 1 of a card in Hearthstone. You may note that these reasons are similar to the article on Magic: The Gathering.

• The card is only good in certain situations. Drawing two of this card would be a bad thing in almost every game, but drawing one can really help sometimes and not hurt too badly other times. This applies to most combo cards (which need the other half of the combo to be good). An example would be Sabotage.
• The split. Sometimes you do want two (or three) of two different, but similar, cards. Each of the two cards is better in a different situation, but both have a similar effect. You run both of the cards so that you have a good answer to either situation and sometimes the best answer. However, you don't want to run all four copies. An example would be Shadowflame and Hellfire.
• The card has a really high cost. Drawing two 10 cost cards before turn 10 counts as two dead cards in your hand. Also, a 9 or 10 cost card is such a high cost that you are unlikely to play it immediately after drawing it because you couldn't do anything else that turn. It is usually best to have only one copy of a high cost card in your deck. An example would be Mind Control.
• A countermeasures card. A card that is very reactionary to a particular play by your opponent only needs to have two copies if the particular play is very common. Otherwise, the possibility of drawing two of these and having them simply wait in your hand doesn't further your own strategy. An example would be Big Game Hunter.
• A reach card. There are two types of reach cards. One, you would only play this card to increase your damage on a turn that you win. Two, you would only play this card in a long game in order to extend your deck past when it would normally fizzle out. Most examples of these reach cards are legendary cards. Despite that two copies are sometimes used, Savage Roar is an example.

In addition to all of these examples, sometimes it is just a matter of not having space for two cards. For example, if I used an odd number of legendary cards in my deck, I'd have to choose to include only one copy of some non-legendary card.

Sometimes you have to run only one card because of the limited numbers of card in a deck. This is mostly due to how much this card synergizes with the rest of the deck.

For example if you want to make a Shaman deck with mana tide totem i suggest you only take one, because you don't want to waste any more slots, better take a card that has more board influence.

Another example is Holy Fire, it is a good card to make damage and heal yourself, but it is high cost and has no board influence, its basically an expensive execute / heal. Many times you will have it in your hand but you won't use it because you have power words or Big game hunter and so on.

That means it is a strong, but very situational card which will prove itself useless in some situation and useful in others.

So in summary, I recommend to only use one when:

• The card is very situational
• It's effect is not always gonna work ( see mind control tech for example, very unlikely to be used twice in the same game )
• You already have similar card and don't want too many of that type ( see starfall for example, 2 swipes and a starfall is usually more than enough to clear board as druid )

I will eventually update my answer it anything else comes to my mind while playing