What would be the correct tape width for a rim of 16 mm internal and 21 mm external?
The rim geometry can be found here (press the right small button on the page, just below the wheel picture):
https://www.dtswiss.com/Wheels/Road-Wheels/R-20-DICUT

not so wide as it covers the tire Bead seats.. what is the distance between them? that wide.

I'd try Velox 11mm tape first, otherwise the 16/17mm should definitely work.

Thanks.
It is around 18 mm (let's say +/-0.2 mm - it can not be measured very accurate inside).
The rim section is curved - should I add some more width for that?

11mm would be too small for sure. A bike shop reccomended me 15mm (that was what they had) and I tested it. I had a big breach of 3 mm on the tube whilst I was inflating it to around 6 bar.

DT recommends 21mm tape, if you're going to run tubeless you'll want the tape wall-to-wall:

https://www.dtswiss.com/Resources/Su...ution-Road.pdf

Yes, I noticed, but that is adhesive tape for tubeless. I use clinchers with tubes. I don't know whether that adhesive tape of 21 mm would work, DT Swiss would not say...

I'm following their recommendations for my R460 rims using that table and it's working very well. I did go with Stan's tape instead of DT tape, cheaper and thinner. I haven't set up tubeless yet, I need to wear out my Panaracer Pasalas first.

BTW, Velox tape sucks, it holds moisture and causes nipples to seize to the spokes. There much better options now, Velox was around in the 70's, it was a known problem back-in-the-day.

I've been using the Velox tape for over 250,000 miles with no problems.

I have seen so many wheels suffer from VIC (Velox Induced Corrosion), why would anyone use it when there's better solutions on the market for decades?

11 mm is going to be way too narrow. 17 mm works on every road bike rim that I've ever tried.