pk,
in theory, you could have the menu as long as you want - the problem would occur if, for example, you embed the menu into a div tag with a limited width... meaning say your wrapping div is 900 pixels wide but your menu contains 11 items and each menu item is 100 pixels... well then as the math suggests, it won't fit and will therefore cause the items that are beyond the width of the div to wrap.
To correct, I would find out, what the exact width of your menu will be - don't forget to add the padding with the width so you accurately determine the length of one menu item and then just multiply it by all items. Then make sure the div and/or other wrapping items are wide enough to handle the width of the menu.
Best regards,
Brian