what is too many indexes

Some of our tables have 12, 14 , 16 or more indexes on them. I have always being told and belived that too many indexes and indexes of the wrong kind can slow queries down i.e. indexes can take up loads of disk space and can become skewed etc .... Unfortunately, I cant find much info in the informix manuals to support this in detail. Can anyone refer me to another manual or source of information where I can find out more about indexes and performance. Thanks.