Answer:
The original version of Excel had a maximum limit of 16,777,216 (16.8 million) cells within a single worksheet and that continued to be the worksheet limit until it was later expanded in version 2007 to the current limit of 17,179,869,184 (17.2 billion) cells.
Originally within each worksheet, Excel provided columns A to IV, which is 256 (2^8) columns; and 65,536 rows (2^16); therefore 256 x 65,536 = 16,777,216 (2^24).
As mentioned, in version 2007 the worksheet maximum size limits were greatly expanded, providing columns A to XFD, which is 16,384 (2^14) columns; and 1,048,576 rows (2^20); therefore 16,384 x 1,048,576 = 17,179,869,184 (or 2^34).