This setting cannot be changed later, but you can always create and populate a new index with a different setting.

This setting is important for scoring. We use this setting as a means to estimate how many other items there are, and therefore how important the given document is for each term in the overall index. You should try to estimate as closely as possible to the eventual results this index is likely to grow to, but with partitioning in mind.

We also use this setting for partitioning. This means that the higher this setting is, the more partitions are created for a given set of terms. As in, for a tiny index, we may just have one "Token Set" for each letter of the alphabet. For a very large index, we may create one for each 4 letter combination (exponentially more).

More partitions can increase cost, because we are writing more items to the database more often. However, searches are also much less likely to get bottlenecked.

For cost-sensitive indexes, we recommend setting this at what you feel you may grow to over the next few months or so, and if growth explodes, you can always reload your index into a clone for better performance. For performance-sensitive indexes, feel free to set this aggressively high.