There are people who likely have an idea of what's going on but who don't respond because something unusual has happened and instead of having a simple way to fix the problem, a solution (which may not work) takes time to explain in a forum like this vs. a tech support person who, if given remote access to the computer, can probably fix it pretty quickly.
I'll give it a shot.
So - I'm presuming you have APFS not HFS+. The "Preboot" volume is one installed during installation. It's there, it takes up space but you don't want to erase it. It's presence will usually disappear before the system completes booting. That it is still there may just be a one-time occurrence (hopefully).
I'm presuming MyBackup is some sort of external disk that you use for backup.
The extra number at the end usually appears because the system is given a command to mount a filesystem at, say /Volumes/MyBackup but that folder location is already taken by either another filesystem or maybe a local directory has been created with that name.
So what you want to do is erase the Preboot ... and MyBackup ... directories but not delete the filesystem associated with it if it is the mount point for a filesystem.
Run the Terminal app and from the menu at the top of the screen, select Shell -> New Command. A small window will pop up. Type in "df -g" (without the parenthesis) and press the "Run" button. The small window will disappear and a new window with the results will appear. Stretch this window out so the lines of text don't wrap and you can see the last column on the right that says "Mounted on" clearly.
If any of the /Volumes/Preboot, /Volumes/Preboot 1, etc. appear in the "Mounted on" column on the right, you do not want to erase these. For the other folders you listed in your OP, check in the Finder to make sure there is nothing in the folder. If it doesn't appear in the "Mounted on" column and there's nothing in it, you can delete that folder. It might ask for the system password to do this.
Disconnect MyBackup if it is an external disk. You can quit the Terminal app. Restart the system and the folders you listed should no longer be in /Volumes. If it is or there's some other problem or if you have clarification on some assumptions I've made - just post.
You mention scripts with relation to MyBackup. If the additional MyBackup folders appear again, it's possible one the scripts you have is causing the problem. It might be that the script works most of the time but under certain circumstances it doesn't. That the Preboot folders appear could also be an issue with a script that's not properly written - it also may be a system glitch - hard to say without looking at your scripts.
