The number of populated cells Numbers can deal with, depends on the installed memory of your System. Look there --> http://support.apple.com/kb/HT1017
A Excel 2007 or 2010 spreadsheet contain 1,048,576 rows and 16,384 columns.
A Numbers spreadsheet contains 65535 rows and 256 colums like the older Excel up to Excel 2003
So you may get an error trying to open a 1.048.576 row and 16.384 columns sheet in Numbers, especially if there are populated cells outside the 65535 x 256 range of Numbers.
Or if the number of populated cells in the sheet you want to open is not supported by your system memory.
Cheers - Lupunus