If you only use one OS, you vote once, even if you have it on multiple machines. We're not really looking for a count of machines here so much as a count of people who use a given OS. So if you have two machines, one with Win 8 and one with Win 10, it would be good to know you use both. Knowing you use Win 10 on both is less valuable.
Up to you whether you want to count the older machines. If you do use them, it's a valid option, but if it's very, very seldom then probably not that useful.
- Oshyan