apparently 600 words, which are either derived from Latin or cognates, for whatever reason.
Edit: I have to correct the above. In the link below it says that 600 or so old Welsh words were borrowed directly into ancient Welsh.
Some of these will have dissapeared and are obsolete, but more will have been added from later Latin - religious and legal connections etc - all the scholars /monks etc etc wrote Latin. Then there are borrowings from other languages like Norman French, as happened in English. Then there are those later on from English, many if not most of which which came from French, many of which in turn came from Latin.
Then there are other words which went into Latin from Celtic (Gaullish and the Iberian languages) - words for Chariot etc. Some words have done a round trip from Celtic / Gaulish or Ibero celtic into Latin, into English and back into Welsh - Car is a good example of a word like that.
The common PIE routes also means that many are simply cognates, perhaps derived in both languages from a more ancient source.
Maybe 600 is a Conservative figure.
N.B. I just read that melin came into Welsh from late Latin.