The FlexReport API defaults to 100 rows.
You can override this in your call by adding a limit parameter set to -1.
Reference the below for an example query:
{
"sqlStatement": "SELECT timeInterval_Month AS Month, SUM(lineItem_UnblendedCost) AS \"SUM(lineItem_UnblendedCost)\" FROM AWS_CUR GROUP BY timeInterval_Month",
"dataGranularity": "MONTHLY",
"needBackLinkingForTags": true,
"limit": -1,
"timeRange": {
"from": "2021-03",
"to": "2021-04"
}
}