If you do not get married within the 90-day period permitted by your fiancée visa, your fiancée must leave the US, and can only reenter the country if you begin the process again, petitioning for a new K-1 visa. The same requirements apply to fiancée visas that are denied.
According to the Immigration Marriage Brokers Act of 2005, a US citizen can only petition for two K1 visas. If the first marriage fails or never occurs, the American petitioner must wait two years to apply for a new K-1 visa, even if it is on behalf of the same person.
In some circumstances, you will be able to apply for more than two fiancé visas, but this will require a waiver, which can be difficult to obtain. Therefore, you are advised to work with an experienced K1 visa lawyer to ensure that your first petition for a fiancée visa is successful, and that you meet all requirements afterwards for your spouse to gain citizenship.