I noticed that before 5 comes 3, and after 5 comes 7. In that part of the world the number 4 is like a black cat with 13 painted on one side and 666 on the other walking under a ladder at midnight. Depending on location, you can find floors enumerated as 4 missing from hotels and room numbers containing 4 missing in hospitals - you just don't mess around with 4 if you can help it.
But 6? Gunter's space page shows little more than a placeholder, and other web sites (see screen shots below) have no mention.
Was it just cancelled, or was there trepidation also because of a quirk of language. I'm told that the number 6 can also sound like a word for "falls down" and so would be particularly unlucky for a satellite, but I am not sure if this is a coincidence, an amusing story, or turns out to be at least partly the reason.
For this answer, I need someone who actually knows the answer or can find a credible source, not just a quick copy/paste/forget. Thanks!
above: from http://www.nspo.narl.org.tw/en/
above: from http://www.nspo.narl.org.tw/tw2015/projects/FORMOSAT-5/program-description.html using google translate in Chrome.
above: suggestions for a recent internet search