With FieldRangeVector() the limit is 1000000, with FieldRangeSet::indexBounds() the limit is 4000000.

The FieldRangeVector() should be a lot more scalable than FieldRangeSet::indexBounds(), so we might consider use of an even higher limit for that case. Currently FieldRangeVector() is used for btree traversal and FieldRangeSet::indexBounds() is used when figuring out which shards an operation should go to.