I got a challenger deck with Sarkhan the Masterless which can create dragons.
In the deck there are two dragons. Do they just count as part as your library and if you have only one you can only create one dragon (given it dies) and if you have 5 you can create 5?
They also have a front and a backside. Can I only use them once?
I think you’re confusing tokens and token cards.
Tokens are game objects that are generated by various spells and permanents. They are never part of your deck and indeed can not exist outside the battlefield. If a token moves to, for example, the graveyard, it vanishes instantly after moving, as far as the rules are concerned.
When an effect tells you to create tokens, you just do it—they exist now on the battlefield and don’t “come from” anywhere inside the game. You can represent them with anything you want, even dice or scrap paper or a basic land with sharpie on it, but Wizards of the Coast also prints token cards you can use.
Token cards are cards that you can use to represent tokens. While they’re by no means the only way to represent tokens (see above), they are by far the most convenient way to do so, as you can use the cards to easily represent various game effects, like their tapped state or whether or not they have counters on them.
Token cards don’t go in your deck. You keep them with you while you’re playing, maybe left inside your deck box. When an effect instructs you to create a token you can take one of those out and put it on the battlefield to represent the token.
You can find token cards from various booster packs, and pre-constructed products. They are not in any way tied to these products and can be used anywhere. Various sites also offer custom token cards with their own artwork. Some are two sided, so you can use the side you need (ie. the side that represents the token that was created). As far as the rules are concerned, the other side does not exist.
Do they just count as part as your library?
No, tokens are not part of your library. Your library consists of cards, and a token is not a card. Comprehensive rules 111.1:
A token is a marker used to represent any permanent that isn’t represented by a card.
For your next question:
if you have only one you can only create one dragon (given it dies) and if you have 5 you can create 5?
You can create as many tokens as effects allow you to. The only requirement is that it is clear to your opponent(s) how many tokens you have on the battlefield at any time and what type each of those tokens is. If you run out of the official tokens, just use anything that’s convenient.
Can I only use them once?
As above, you can use them as many times as effects allow you to.
The pieces of cardboard included with your deck that represent tokens (such as a dragon) are really just memory aids. You can put them on the field to represent tokens that you make, but you’re allowed to use anything to represent your tokens, as long as you and your opponent are clear what each thing is. You could use flipped over cards from outside the game, sticky notes, Pokemon cards, index cards, scraps of paper, dice, etc.
For instance, if you are able to use Sarkhan’s -3 ability five times over the course of the game, you would create 5 dragon tokens, even if you run out of dragon token cards to represent them with. Some decks can spew out tens or even hundreds of tokens, and it would be impractical to require a card every time.
You can reuse these token cards any number of times. Similarly, if you have a token card that has a 5/5 Dragon on one side and a 1/1 Servo on the other and the 5/5 Dragon side is currently being used, you aren’t prevented from making Servo’s. You can still make a Servo, you just have to use some other object to represent it.
Tokens are not part of your library because tokens are not cards. You can create as many tokens as effects allow you to so you can use tokens more than once.