Pretty much there's two schools of thought with retinues. The first is the one most of us go with-using retinues as a complement to your regular levies or otherwise building them in enough numbers so they stand on their own. The second is building cheap, low quality light skirmish retinues purely for their intimidation factor to keep your vassals in line.
With that being said, if you're going with the former then think about your culture. Do you happen to be something like Norse and Irish with heavy infantry, Andalusian and Berber with their light cavalry, or Scottish and Italian with their godly pike retinues? If that's not the case, are you something like Greek, Turkish, Persian, or French? Heavy cavalry and horse archers aren't exactly the most bang for your buck though with your high income you might be able to get away with it. If you're outside of some of the cultures mentioned, then a mixture of generic retinues is more than likely the answer.
Beyond that, quantity is definitely a good thing regardless of your school/strategy though do of course consider what you can actually afford. One lost battle or some bad attrition might end up costing you a pretty penny.
