
Feylin |

All items hae a base of 2 challenges - one at the very beginning, one at the very end. In addition, there are a number of additional random challenge - 1 for every 5000gp in the items buying price (with a minimum of 1). So all magic items have a minimum of 3 challenges.
Thanks but where does it say that?

Jeraa |

The magic item creation system in this section divides the creation of magic items (other than potions and scrolls) into a series of challenges that the creators try to overcome. These challenges represent either setbacks or opportunities in the course of the creation process. The first and final challenges in the process are the same for every item: preparing the vessel and completing the item.
Between those steps, the characters face a number of random challenges based on the total market price of the item. The process includes one additional challenge per 5,000 gp in the item's market price (minimum 1), though the GM could increase or decrease the number. Having more challenges means it's more likely that an item will have unexpected properties, for good or ill. It also makes the item's creation more expensive or time consuming on average for less skilled characters, and conversely the process will likely be cheaper and faster for a highly skilled party.

Jeraa |

okay now one more question. So say I am missing a spell to make an item. So I add +5 to the check or is that only on normal magic item crafting
That is for normal magic item creation. The final challenge in dynamic item creation is the "Complete the Item" challenge. The two options for the task for that challenge are:
Improvise Use Magic Device DC 15 + item's caster level
Provide the Requirements Meet all the item's prerequisites
So if you meet all the prerequisites, you automatically succeed on that challenge. If you don't, you need to improvise and make the check.