The max cycles possible depend on your device. It’s all just trial & error.
Regarding fixed vs max cycles: sometimes, especially when playing older games, fixed cycles will give yöu a better gaming experience, as cycles=max can simply be too fast. Ultima 6 or Wing Commander 1 for example become unplayable if Dosbox is running too fast. And while cycles=max will give you the best performance possible, it can also be inconsistent, because it’s relative to the cpu power assigned to Dosbox.