As all pre-modern East Asian society, polygamy was accepted.

However, in all likelihood, only the richest members of society could afford to have more than one wife. Meaning daimyōs did regularly. Their wives would usually be daughters of other daimyōs for alliances or daughters of aristocrats for access to Kyōto court and politics and prestige. Concubines seem to have been more often daughter of vassals (or of defeated rivals). Aristocrats though, being the time of great financial hardship for them, rarely took concubines it seems, and even the imperial household seem to decrease the number (though interestingly lots of women got to work as official court staff in positions that used to be for men).

The majority of the population, including lower samurai, doesn't seem to have taken concubine nearly as often though. A second wife was costly after all.