I would say to take a look now at the other machines you have and see if they are still in good condition or if they are getting rusted too. Then you know if you need be concerned wit that or not and can plan accordingly.
I don't keep back up machines for parts. I strip parts off and get rid of the body. I just don't have the room to store a bunch of parts machines.
For me, if I didn't work on them or have the time to work on them I would have far less. I would choose some I love to sew on and a few I love for my collection but the rest would be pretty overwhelming. I don't personally stress about the number of machines I have that I have worked on and are in good condition. Even if they sit awhile and need an oiling, that's really all they need and they don't stress me out. If they did I would find homes for them.
The ones that can stress me out are the ones I havn't had time to work on yet. If that's the condition most of yours are in and you have no time to work on them, I would probably re-home them.