How many of the definitions in dictionaries generally are really used?
Wiktionary seems pretty good about marking their definitions.

Take the character 士 for example. 


Entries 1-4 are marked with the footnote:



  Obsolete sense in Modern Standard Chinese - may be preserved in fossilised compounds and expressions or other varieties of Chinese.



Entry 5-7 are marked:



  (historical)



Entry 8 has no markings.
Entry 9 is marked both with an:



  (honorific) 


tag, and the same footnote from entries 1-4.


Entry 10 is marked: 



  (Chinese chess).



Entry 11 also shares the footnote from the first four.
Entry 12 is mark-free.


That basically only leaves entries 8 and 12, as possible meanings in Modern Standard Mandarin. While, entry 12 is, itself, a surname - just leaving entry number eight as the only meaning you need to really remember. Other compounds and expressions with the character can just be treated as single entities without needing to worry about a specific characters meaning.