What time of year you are going will also impact the number of days you need.
Peak times- 3 day park to park tickets. Universal is two parks with entrances literally right next to each other <5 minute walk from one park to the other. You can use thus to your advantage by looking at ride time on the
Undercover Tourist app and hitting rides with typically long lines when the are at low points of the day.
Off season times -2 Day park to park ticket would be enough. Most likely done in 1 and 1/2 days and you can use the extra 1/2 to hit your favs again. I went once for two days back in 2012 in February during a family visit to orlando (not vacation) and the lines were very short for most things.