Semen shipping container quantities

How do stallion owners determine a safe number of equitainers to purchase?

After all my research I am thinking I would prefer to buy equitainers even though they are expensive, mostly because I don’t agree with disposable containers and all the waste they create (and I hate how stallion owners keep reusing them way too much) but I have no clue how you decide how many to buy. I am very much a numbers person but I just don’t know how to logically calculate it given how many factors affect it seeing as all mares are cycling at random times, what if lots of mares all suddenly need semen during the same week, what if somebody takes too long to return one etc. I feel like I would be still wanting to buy some disposables for just in case and then I guess play it by ear as to whether additional equitainers are needed, especially as the stallions get more known etc