Just a short question is 18mm enough to cover the rim entirely or do I need a bigger size like 21/23mm?
If you are using a tubeless tape (whether or not you'll be setting the wheel up tubeless), going 5mm-6mm wider than the internal width is a good rule of thumb. Inner width plus 5mm is ideal if you have that exact size tape at hand, but if you don't then it usually works better to go a little wider than narrower. There's some variation among tire and rim manufacturers in what they recommend generically - for example, Stan's says inner width +2mm and WTB is inner width +5mm, even though their rims and tape are both very similar in design. I tape a lot of random and OEM tubeless rims, usually with little time to deliberate, and usually just do the +5mm, which is also usually basically the same as the outer rim measurement. A little bit riding up the inner sidewall is okay as long as you can get it to conform neatly and it doesn't curl on to itself.
For non-tubeless tape, all you usually care about is will it cover the spoke holes and sit neatly in the rim well. Some low-end rims have vent holes or other rough spots that need to be covered as well. Most non-tubeless rims have a range of tape widths that can work, but I usually go for whatever width goes from sidewall to sidewall on the inside, such that it can't possibly squirm out of place and leave a spoke hole uncovered. This is usually a few millimeters wider than the inner width measurement (since the inner width is going from hook to hook). 18mm tape on a 17mm internal will usually be good. The main exception is some rims in the world have a more defined channel type area in the middle where the tape is intended to lay, so use whatever neatly fills in that channel for those rims.
This is a personal preference. You certainly need to go wider than inner rim width. This is to account for:
- The channel in the middle of the rim
- The tape stretch. Some tapes are "stretchier" than the others and as you pull on the tape it will become longer and narrower. The effect is minor but not negligible.
There is no "rule of thumb" to decide how much wider the tape should be, but I would say 18mm is too narrow. 20 should be OK.
Take the tires into consideration. If you will be running tubeless, then you don't want the rim tape to go onto the rim sidewall where the tire/rim interface is. If you run inner tubes, then it is perfectly normal to have rim tape "too wide and overlapping the rim sidewall a bit. For this last scenario it is in fact much easier to apply the oversized tape...