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.