Why isn't more always better?
Sets create both stimulus and fatigue. Additional work is valuable only while it contributes enough useful stimulus to justify its recovery cost.
Do compound exercises count for multiple muscles?
Yes, but not always equally. Bench pressing meaningfully trains the chest and triceps, for example, but that does not mean every muscle receives an identical stimulus.
When should weekly sets increase?
Consider increasing volume when progress has stalled despite appropriate effort and recovery and the current workload is being tolerated well. Avoid changing several variables simultaneously.