Does anybody know how many words in the OED have the remark "of unknown origin"?. I read it often, but have no idea how to find an answer. I guess one would have to ask the editorial staff of the OED.
Take a look at the 1981 study by Sterling Eisiminger, “Etymology Unknown: Toward a Master List of Words of Obscure Origin”, and his 1984 followup, “Etymology Unknown: The Crème de la Crème de la Crème”.
The author of the study wrote to Merriam Company and found that according to their dictionary there are 538 words of unknown origin. The author further goes on to cross check this list with a dozen other dictionaries (including Oxford English Dictionary) to narrow it down to 84 words that are of near mysterious origins. To quote,
They (the 84 words) are the crème de la crème of English etymological mysteries.
The papers were published in 1980s. The first paper contains the full list of the said 84 words and the motive of the author was to spur etymological research in those obscure words.
Quite literally, there are 910 entries in the OED that contain "of unknown origin". You can see this by searching "of unknown origin" in the online OED. If you expand the search to also include results that contain "unknown origin", there are 2207 entries.
Many of these entries have not been updated. Also, for most of these entries, there is something known about the etymology. For example, the page for aal (the first result) says it's from:
Hindi āl Indian mulberry tree, of unknown origin