You might have accidentally deleted it. For new ones, I recommend either making one yourself in photoshop or going to Osu Skinner Website.
Go to the Interface tab, then scroll down until Score and Combo Numbers as well as Score X. Both will be needed. You just need to click on the zoom button, then Download this element.
It's easier:
Do you have elements for the combo counter in your skin? If I remember correctly they're having "combo" in their names (for example: combo-1, combo-2, combo-x, ...).
If you don't have them, make sure it's ComboPrefix: score in your skin.ini.
If you do have them, it should be ComboPrefix: combo.
Make sure to press ctrl+shift+alt+s in osu! after fixing.