FAR guidance regarding IDIQ contracts may be found in 16.504. The information required to make intelligent contract formulation decisions is derived by a thorough planning phase. Current strategy and stakeholder input is analysed and informs the planning process. Current market research and supplier identification further informs the analysis.  The contracting officer then considers the following when determining the number of contracts to be awarded:

(1) The scope and complexity of the contract requirement. (2) The expected duration and frequency of task or delivery orders. (3) The mix of resources a contractor must have to perform expected task or delivery order requirements. (4) The ability to maintain competition among the awardees throughout the contracts’ period of performance.

Applying data analysed in the planning phase, the contracting officer makes the determination as to guaranteed minimum amount and the number of awardees. Awardees receive the same minimum guaranteed award.  If the contract provides government ability to off-ramp contractors,  it is possible contractors failing to provide competive proposals can be removed from the IDIQ and future TO consideration. Meeting the guaranteed minimum award would be required if it is included as part of the contract.

Reading GAO cases is very helpful to better understand real examples of fair opportunity being done right, and when it goes wrong. You can find GAO cases at www.gao.gov and searching for the key search term "fair opportunity".