Since we no longer have a monolithic index that needs to hold all historic data, scaling up or scaling down according to the recent volumes becomes easier. The choice of the number of shards is not an upfront and permanent decision. Start with an initial estimated number of shards for the given time period. This number, the chosen number of shards, can be put in the index template.

Since that choice of shards can be changed before the next timeframe begins, you are not stuck with a bad choice. With each time period, it gives a chance to adjust the index template to increase or decrease the number of shards for the next index to be created.