The following worked for me.
$ splunk search 'index=_internal | stats count' -index_earliest 1493132552 -index_latest 1493404081 -sample_ratio 1000
INFO: Sampling disables usage of report acceleration summaries.
INFO: This search is sampling approximately 1 out of every 1000 events (seed=1317954456)
count
-----
58
$ curl -sku user:pass https://localhost:8089/services/search/jobs/export --data-urlencode search='search index=_internal | stats count' -d output_mode=csv -d earliest_time='1493132552' -d latest_time='1493404081' -d sample_ratio='1000'
count
46
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