During the heyday of the condottieri many Italian powers did their wars more or less entirely through mercenaries. And armies of out-of-work mercs were a severe problem for law, order and pretty much everything else in northern France during the lulls of the Hundred Years' War - some of them went to Italy or other places to find work, others just turned to banditry. At worst cases there were veritable armies of such marauders making life miserable for everyone and requiring full-blown military campaigns to get rid of.

As a rule of thumb, everyone who could afford it augmented his other forces with as many mercenaries as he could afford, especially if they had competences the local troops were deficient in. You can't have too many advantages in a war after all. In other locales the kings and generals by and large had to do without, simply because they lacked the solvency to hire outsiders to take part in their wars - although you could also try to make like William the Conqueror and offer the hired warriors estates in the lands to be conquered (which also did wonders to their motivation), if your planned campaign was like that.

In other words, varied like Hell. We're talking about an entire subcontinent of very diverse local conditions here, for a timespan of some five hundred years - hard to say anything too specific about that kind of coverage.

Americans are probably familiar with the somewhat peculiar phenomenom of state mercenaries, regular army soldiery hired out by their ruler - the Hessians of the American Revolution fame were this sort, sent over by the Prince of Hessen due to dynastic ties with the British royalty and good old cash payments. Not a common practice, but that probably wasn't the sole example either.