So, as you are aware, for the best case solution you should be using update product amendments that change the quantity on the date specified:
Monday, create subscription set quantity to 50, invoice for 50 for a month.
Tuesday, update product amendment, set quantity to 1050.
Wednesday, update product amendment, set quantity to 1020.
A month from Monday, invoice for 1020. If you didn't invoice on Tuesday or Wednesday this invoice will also include proration charges for the Tuesday & Wednesday changes, or you might have invoiced back on Tuesday and Wednesday (our recommendation - get the money as soon as you can!!!).
In your original posting you expressed concern about having so many amendments bill performance will suffer. Which it will if you approach the documented limits, but I'd ask how often that will be the case. If you have 1000 subscriptions how many would be so volatile you could reasonably expect to approach the limit inside 12 months? I'm always wary of situations where the tail is wagging the dog, meaning while you might have a handful of subscriptions that do head up towards these limits, if the bulk don't, I'd stick with the update product amendment approach as you'll collect more money, quicker and accurately. For those subscriptions that do start to drag on performance, create a process (may be automated, may just be manual) where you clone the subscription, cancel the old one, and start the new one of the next billing day with the then current quantity.
But if the alternative is true, you're herding cats and everyone's subscription is collecting 30 update product amendments a month (one update product amendment a day), then you probably have about a year's worth of use out of that subscription before you collect 300+ amendments and it may be worth duping the subscription as described earlier. If this is the case I'd automate the duping process mentioned earlier BUT STILL stick with the update product amendments process!
Please remember I'm not at all familiar with the dynamics of your business but you're paying for a platform that's quite scalable, is continuing to scale even more and is automatable meaning you can create automated processes to work around the issues you do come up against.
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