Do you have a rough number of creative assets that you aim to make per $15k of spend?

This is probably not the best way to think about how to make creatives. I would mostly recommend the multi-armed bandit approach. You start with a couple creatives, run campaigns on those, keep the good ones, and continue iterating on why those are good.

Creatives ads are one of the most crucial aspects of performance marketing. It's important to adapt on the go and do not stick to a specific amount of X creatives per $Y in media spend.

It very much depends on which stage you are in soft launch, are you trying to maintain the spend or scaling up? How fast are your creatives are downgraded or need to be refreshed? how many channels you actively promoting? Is the 15K daily, weakly, monthly budget?

I'm very hesitant to state a number of X creatives per $Y without context, however, without a performance baseline, I'd recommend starting with around 5 concepts and create variations of the best-performing ones.

I'm not sure that "X creatives per $Y in spend" is the best way to look at the creative production cycle.

There definitely is a cold start problem with creative at launch: you need some set of creatives to launch a new app with, and you want enough of those to ensure that you've explored / experimented with a number of different themes and don't get stuck at a local maxima with respect to performance. But after that -- once you have some baseline performance data and you are either trying to maintain spend or scale it incrementally month over month -- then you can intuit the volume of creative you need on a weekly basis just by watching campaign performance.

I wrote about the ad creative lifecycle and the performance "half life" of ad creative; if you need to assign a volume of ad creatives by $X spend for planning purposes, you can back into that number just by watching for how quickly your ad creatives start to degrade and using that to come to the half life value. But the reason I'm reticent to say it's useful to think of creative production in dollar value terms, at least as a generalization without any context, is that I've seen some creatives for very broadly appealing apps stay performant at meaningful spend for a very long period of time but then degrade very quickly when spend was doubled or tripled. So how do you capture that in a X creatives for $Y spend heuristic? Just like overall campaign performance, creative demand tends to change non-linearly with spend.

In general, though, I do think marketing teams underinvest on creative -- it's probably the biggest source of performance frustration I see when talking to marketing teams.