Framework contracts you generally want to specify options for more than you think you need as it locks in the price, the supplier is then contractually required to offer you the same price for later 'options' orders as they did for early ones (adjusting for inflation) until the potential options have been exhausted and then you can either negotiate additional options or agree a completely new contract. Otherwise after youve ordered 200 and you decide you need another 20 they can turn around and say they want a 30% higher price and your at that point locked in as your logistics, maintenance and training all revolve around that model. At the same time you dont want to go crazy and say you want 10,000 options but only place 50 firm orders, both because your endangering your relationship and endangering the financial security of the supplier and secondly as it will be reflected in the price, they need to ensure they have the capacity to meet any potential order which means ramping production and a large order means extra production capacity and staff to be hired and trained which will be reflected in the price, i,e, if you say you want potentially 50 a year for the next 10 years and they have an existing factory that has enough spare capacity or with minimal investment is capable of achieving that capacity then you should get a good price, if they need to potentially double capacity and build another two factories that will mean those costs will be reflected in the unit price.
The downside of framework contracts is it locks in a certain current specification and disincentives improvements as better but more expensive parts or additional features wont be reflected in the unit price unless a variation is negotiated and the manufacturer are also incentivised not to from economies of scale, down the line this can lead to a stale design not taking advantage of technological advancements over the last 10/20 years, continued use of obsolescent parts that would otherwise have been replaced with newer, easier to source equivalents, and still paying for functions to be included that you no longer require (e.g. controls, communication/signalling equipment, interior furnishings that you have since refit the rest of its siblings to replace) that the operator then rips out and chucks as soon as they take delivery of the unit.