From the docs it wasn't clear to me that the number of index entries for an array field is based on the number of distinct values in the array.
I read the docs below to imply that the number of entries is based on the number of values in the array.
For example, this array [1,1,2] will generate 2 index entries, not 3. This was highlighted in NHTT training and I think the docs should explain it.
"To index a field that holds an array value, MongoDB creates an index key for each element in the array. "
"MongoDB creates separate index entries for every element of the array"