If I understand you correctly, you are looking to create a report that will have the reservation information for all reservations with a check in date between an earliest and latest date. While you can't select a latest check in date, you can select an earliest check in date and a latest check out date before exporting reservation data.
This should narrow down the export to roughly the range you are looking for. Once you have the CSV file, you can sort the rows by the check in date and remove any rows at the end that have check in dates past the latest check in date that you desire in the report. This should leave you with only the reservations that have a check in date between an earliest and latest date.
If you simply desire the total number of reservations with a check in date between an earliest and a latest check in date, you can still follow the steps I described above to create the spreadsheet. Once you've done this, you can look at the row number of the last filled-in row of the sheet. Subtract one from that number (for the sheet's header), and you should get the total number of reservations with a check in date between an earliest date and latest date.
Finally, you can also purchase a reservation data import service (we're happy to do a custom export instead of an import) through us to generate the report you desire.