I searched for it and found the link C# EPPlus OpenXML count rows
int iRowCount = currentWorksheet.Dimension.End.Row - currentWorksheet.Dimension.Start.Row;
There is a way to get the cells that don't have no value using the Where method. We can use it to find the very last cell (and its row in this case) from a worksheet, or a range.
int lastRow = sheet.Cells.Where(cell => !cell.Value.ToString().Equals("")).Last().End.Row;
About the Where method : https://msdn.microsoft.com/en-us/library/bb534803(v=vs.110).aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-1