Let's say as an example that your numbers are in the range A2:F6001
You can then use the COUNTIF function as suggested by Da_Taxman to count all those cells that contain the number 7, a formula that results in the number 7, or a string that when evaluated as a number results in 7:
Depending on your locale (language) you may have to use comma instead of semicolon in the formula.
If you want to also count numbers that are the results of calculations and which end up very very close to 7 (e.g. 6.99999999999), but not exactly 7, then things become much more complicated.