You can order parts today that you'll need in four, eight, twelve, sixteen months. You can do that even though both you and the competition plan to release new models in a few months, if you're willing to decide on the storage capacity of your next product already.

So how much you need is a forecast, or more precisely, a range of forecasts that depend on what you and your competitors will sell. Your order will say "I commit to buying x and want an option on y more" and a larger x gets you a lower unit price for the first x you buy.

If the vendor's production capacity is within your forecasts, or near it, why shouldn't you just commit to order everything and get a nice low unit price? You'll lose if your forecasts are very wrong, but then that's true whatever you do. It sucks for your competitors, who did not order according to their own forecasts.