All else held equal, while more strings in the string bed can certainly have it's benefits, it can also be a two-way street, with a mix of pros and cons, namely:
Pros:
+ More control (especially on flatter strikes)
+ Better feel/touch (from higher density)
+ Usually more consistent string bed behavior over a larger percentage of the face area (especially out towards 9 and 3)
+ Better slice control/predictability
+ Possibly better spin, if/when you derive it more so from surface friction (as opposed to ball bite / snapback from more open cells)
Cons (usually cons, but not always):
- Less free power (from less trampoline/cupping/sling-shot effect)
- Potentially less free spin (if you derive more of your spin from ball bite and open cell based snapback)
- Lower launch angle
- Much less for free in general at zero to minimal racquet head speed
So a lot of potential upsides, but generally, you need to be more sound in your finer-grained ability to control the racquet face, as well as your ability to consistently supply a minimum level of racquet head speed, as you'll typically get much less for free on the low end of the effort spectrum -- thus why 18x20's can often be a no-go for many (most?) rec players versus a properly-proportioned 16-main or 18x19. That said, there are more "modern" (open) 18x20's that have more of the pros and fewer cons -- the Gravity Pro/Tour, Speed Pro, WhiteOut 18x20 and Prince Tour 100P being the main five.
Hope that helps give some perspective.