hoge.html
<table id="xx" border="1">
    
</table>
<script>
function RowCellLength(table){
    console.log("row-length:", table.rows.length)
    // console.log("cell-length:", table.rows[0].cells.length)
}
function createTable(table, dates,rows) {
    newRow = table.insertRow(rows)
    for (let i=0; i<dates; i++) {
        newCell = newRow.insertCell(0)
        newCell.colSpan = "1"  //colspan="1"をセット
        newCell.rowSpan = "1"　//rowspan="1"をセット
        newCell.innerHTML = "new cell"  //セルの中に値を入力
    }

    newCell = newRow.insertCell(0)
    newCell.colSpan = "1"  //colspan="1"をセット
    newCell.rowSpan = "2"　//rowspan="2"をセット
    newCell.innerHTML = "hogehoge"
    newRow1 = table.insertRow(rows+1)

    for (let i=0; i<dates; i++) {
        newCell = newRow1.insertCell(0)
        newCell.colSpan = "1"  //colspan="1"をセット
        newCell.rowSpan = "1"　//rowspan="1"をセット
        newCell.innerHTML = "new cell"  //セルの中に値を入力
    }
} 

var members = 10
var dates = 10
for (let i=0; i<members; i++) {
    createTable(xx, dates,2*i)
}

</script>