The installer is multi use, the licences aren't. You can use the installer as much as you like, provided you have the licences to cover it.
If you upgraded your current pc from a previous version of windows, or it came pre installed, then your licence is tied to your pc. You will need a new licence to install Win 10 onto your new pc.
If you bought a box copy of Windows 10 then you can install that on your new pc, it just can't be used on more than one pc at a time. So you couldn't use it on old or new pc at same time.
hope that helps