What version of Excel are you using?
Use conditional formatting
Assume the range of numbers is A1:A20
For Excel versions prior to Excel 2007 (but it will also work in Excel
2007):
Select the range of cells, A1:A20
Goto the menu Format>Conditional Formatting
Select the Formula Is option
Enter this formula in the box on the right:
=A1>=LARGE(A$1:A$20,10)
Click the Format button
Select the desired style(s)
OK out
For Excel 2007
Select the range of cells, A1:A20
Goto Home tab>Styles>Conditional Formatting>Top/Bottom Rules>Top 10 Items
Fill in the info, select a format
OK out