I have a freinf who is a few stands short of the entire Ultramarines chapter.
Without supporting formations, a codex chapter consists of 5 Terminator formations, 18 Tactical formations (technically 18 2/3), 11 Assault formations, 11 Devestator formations, and 5 Scout formations, totalling 12,125pt.
Guard Regiments are much easier to collect, generally consisting of two or more companies of like formations (Infantry, Artillery, Tank, Heavy Tank, or Recon). An average size tends to be five companies.
An average Regiment of Seigemasters with support might look like this:
While a Regiment of Minervans might look more like this:
A Steel Legion Armored Regiment with support might look like this:
While a Vanaheim Regiment would look like this:
