I think this is what you want, you can append the count to Sheet name like below:
int count = 0;
foreach (string img in filesindirectory) {
count++;
ExcelWorksheet worksheet = package.Workbook.Worksheets.Add("Sheet " + count);
}
If you want to hold reference to the worksheets for other use you can use an array:
ExcelWorksheet[] worksheets = new ExcelWorksheet[no. of images]
int count = 0;
foreach (string img in filesindirectory) {
worksheet[count] = package.Workbook.Worksheets.Add("Sheet " + (count + 1));
count++;
}