One of my VMs ran out of space because of logs... So how much storage maximum do logs need?

It depends what you log, if you log everything you may encounter out of space issues. Logrotation is being executed on a daily basis, 6:25 am per default.
/var/log/vyos-rsyslog for instance is being rotated every day. You can do a 'du -sh /var/log/*' to find out what file became very big and then it depends what's in the file etc.

Can I set the number of old instances to keep?

Of course you can:
'set system syslog global archive file <number>'

or in case you have specified your own files:
set system syslog file test archive [size|file] <number>

Sorry, I need to document that, but haven't had the time yet.

Thanks for your help!

I found it in the corresponding documentation: https://wiki.vyos.net/wiki/Logging

Might be helpful to link it the documentation on the main project page (vyos.io).

Yeah, but I gotta check if it's still correct. We are using rsyslog now and I rewrite the entire thing, I kept in mind staying backwards compatible, but I also introduced a few minor features. Later on, I was thinking to add options for logging directly into databases, right now you can only send via syslog to another server. Also I fixed option which never worked in the original implementation. For instance you can create a debug user, once that user logs in, he sees all messages on the screen etc.

If you however find a bug, please open a bug ticket here, I grab it as soon as I see it.

Sounds awesome. Keep up your work!