I am going through the initial Marketo setup and was wondering what are some best practices in choosing cnames. Does the cname impact SEO? Are we able to create multiple cnames?
The hostname(s) you choose as your CNAMEs won't impact SEO. There are some common ones chosen for easy memorization -- lp.example.com, go.example.com, pages.example.com, try.example.com -- but it's up to you.
All your LPs will be available under all every CNAME, so you should take care to only advertise a single LP under one CNAME so you will not appear to be spreading duplicate content.