How many transistors are enough?

Erik_Squires gave you a good summary of the advantages of multiple transistors.  Another advantage is that, for any given output, each transistor can be run more gently which will improve longevity.

There is a possible downside to parallel, multiple transistors vs. just a pair per channel.  Some designers believe that multipole transistors will muddle the sound and reduce transparency.  Dartzeel once sold a $20,000 amp based on this idea that had only two transistors per channel.  They have since upped the count to four per channel in their $50,000+ updated version of this amp.  Nelson Pass' First Watt amplifiers also use a minimum number of transistors in both input and output stages.  I've heard several First Watt amps, and I had their J2 in my system for a couple of weeks (borrowed from a friend); it was one of the better sounding transistor amps I've heard (I am mostly a low-powered tube fan).

see nelson pass’ extensive comments and writings on his various first watt amplifiers - go to the first watt website

short answer is it depends... on the power delivery sought, the circuit, the characteristics of the particular transistor being used and so on

design of anything is all about working with your raw materials and parts, your objectives, and making smart tradeoffs

Transistor count is incidental to good sounding designs and builds.