James clavell's shogun, and another user's post mention this, but I have never heard of any other mention before. Could someone help me out? Maybe regarding the prevalence, particular use and battles, as well as the reason for samurai particularly.
This is anecdotal, but as of 1623 the Vereenigde Oost-Indische Compagnie (Dutch East India Company) employed several Japanese rōnin in their post in Amboyna, Indonesia (present-day Maluku), presumably as guards. I also am interested to learn more about the spread of these mercenaries, based on this tantalizing detail.
Sorry I've been busy lately. Take a look at this thread. In the Sengoku there's basically little to no difference between rōnin, nobuseri/nobushi ("wild warriors") and mercenaries, at least until you were officially made a samurai from some notable deed in battle. So we should probably consider they were quite common. You might not technically consider them samurai, but there was no clear legal boundary yet anyway. The Toyotomi side at Osaka was made of almost entirely of hired rōnin.
Some famous ones are the forces of Negoro temple (who double as "warrior monks") and the forces centered at Saika in norther Kii. These were know for being arquebus specialists and fought both for and against Nobunaga during his campaign in the Kii penisula like at Noda/Fukushima (where according to the Shinchōkoki they apparently brought 3000 arquebuses), and Ishiyama Honganji, though in the end they sided with his enemies. But large groups like the Saika and Negoro are a bit harder to classify, in that they de-facto also ruled over territory and were also traders. We can also lump some of the coastal small daimyō and their private navies in here as well, depending if you want to count them as daimyō or pirates/privateers (or merchants).
One more thing is that lots of Japanese were bought as slaves and/or mercenaries by the Dutch East India company, at least some willingly. You can read about them here. For /u/Qafqa as well.
After about 1620, there were no more wars to fight, and the Shogunate wrote down strict laws dividing the realm, disarming the temples, and banning Europeans from buying Japanese (mercenaries or otherwise), so mercenaries disappeared (unless you count temporary soldiers/law enforcement occasionally temporarily hired by daimyōs).