Sometimes 1 cymbal is enough.
It depends on the type of cymbal and what you're trying to do.
A right hand crash is both practical and gives more options.
When a crash is too much a splash is nice.
Many jazzers do everything with 2-3 rides and if it's cymbals with a wide range and they're crashable they can be used for a lot of things. Many rockers do the same thing.
4 cymbals will provide enough options for most pure stylistic scenarios, but the choice of those 4 cymbals may not be enough for any style or situation. Still with the right choices suited for your own style maybe it could.
Cymbals are personal.
The answer is the same as for the kit.
Can you do the job to your (and hopefuly your employer's) satisfaction with your choices.
It's natural to share the pic of my setup yesterday again.
Putting aside that it's more or less a rip-off of Vinnie's kit I hardly ever bring the whole thing.
These are all A Customs which are nice works with everything tipe of cymbals, but they have limited range, the ride isn't really crashable and so on, so a 12" and 15" gives options there.
I played a wide assortment of styles. Some required a typical modern pop/ rock approach, some was different types of traditional jazz and some was me going "tribal" on the pandeiro. The big crash was neede for some things as a crash, but mainly it's there for mallet rolls.
This was a job for someone else trying to fit their requirements.
Doing the baisc job is one side of it, expressing yourself is another. Everyone does something different.
REPEAT:
Can you do the job to your (and hopefuly your employer's) satisfaction with your choices.