For a well-configured environment, the answer is "all of them". HFs aren't specific to a specific index or indexes (at least they shouldn't be). The endpoint forwarders should be round-robining (is that a verb?) their data across all HFs for good data distribution.
If you're trying to decide how many HFs you need for the new deployment then plan on twice the number of indexers. The ideal number, however, is zero, meaning endpoints send directly to indexers. I realize, however, that that is not always feasible.
If this reply helps you, Karma would be appreciated.