I can imagine quite a few adjectives related to some small numbers at least, mostly by how they work in my native language (there will presumably be many more). For example with 2 (derived from dva – two, druhý – second, or dvojka – the digit two):
- dvojný (dual, double in nature): dvojná vazba (double bond), dvojné číslo (dual grammatical number)
- dvojitý (double in count, duplicate): dvojitý vrchol (double peak), dvojité béčko (double flat accidental)
- dvojí (twofold) as in The importance of this discovery is twofold: ...
- dvojkový (binary): dvojková soustava (binary numeral system)
- druhotný (secondary): druhotná obava (secondary concern), druhotný přízvuk (secondary stress)
With the exception of the latter, they all are used (with rapidly decreasing frequency, of course) also with 3, 4, etc. And the latter does work in English so there's no fundamental reason for it to be exceptional either. Yet in Duolingo, I have only learnt of adding -a to form ordinal numerals (first, second, ...) as if that was the only adjective paradigm I could ever want to use with numbers.
I have tried to look up some in ESPDIC but found binara or duala (which might or might not come from du but feels more like a direct loanword and I have my doubts that -al- would work with other roots). It's still possible that these are just inventions of people who used a familiar root borrowed from another language and did not try hard enough to find a “native” Esperanto affix that would do the same. Are there any? I would find it very strange, especially for Czech, if there was a niche in which it was so much more regular than Esperanto.