Does that only count words that entered English directly from the various Celtic languages?An oft-made claim (now thought to be somewhat exaggerated, but still rooted in reality) is that there are less than 20 words in English that are Celtic imports. Mostly they describe geographic things that the Anglo-Saxons weren't already familiar with (being from Flatland) such as "crag" and "cairn".
Some of the 'French' words that came over here, such as "mutton", are actually Celtic (Gallic to be precise) in origin.