Quite a few. Particularly iMacs as they seem to outlive their notebook counterparts. My own machine is running OS X Yosemite, and it’s still as capable as ever.
macOS Sierra and macOS High Sierra are just as susceptible to malware as Yosemite or Mavericks, primarily because instead of exploiting vulnerabilities in the system, malware often relies on deception to convince the user into installing the software manually. MacKeeper for example tries to convince the user that it’s beneficial for the performance of the machine. Other malware like Genieo and VSearch come disguised as Adobe Flash Player updates. These run within the user space and are the most likely employed method to catch out a user and take their sensitive information.
Other vulnerabilities, like oversights that allow for unrestricted access to protected memory spaces or allow arbitrary code to be executed, are in theory more prevalent on older operating systems than newer ones. That said, newer systems and software updates can have bugs and vulnerabilities of their own, so some degree of vigilance is required whenever sensitive data is involved even when running the newest software.
Of course it depends on what the machine is used for, but generally speaking for a daily use machine where the user is relatively knowledgeable about the risks associated with being online, where not to be and what not to click (and install), I would have no qualms about using an older machine with a somewhat older OS.