Apparently there is a big number of TMs with the word SEO in them, and some companies have a TM on the word "SEO" alone.
No. This is utterly incorrect.
Folks, there are a LOT of things in the USPTO database - not all of them are registered trademarks.
There is one registered TM for "SEO" as a mark for serving as a human resources department for others. That mark has nothing to do with search engine optimization services.
There is a handful of registered marks including "SEO", and all but one of them specifically disclaim "SEO" from protection apart from the mark as a whole.
SEO UNIVERSITY (and design)
Registration Number 2933018
Disclaimer
NO CLAIM IS MADE TO THE EXCLUSIVE RIGHT TO USE "SEO UNIVERSITY" APART FROM THE MARK AS SHOWN
SEO LOGIC
Registration Number 2734898
Disclaimer
NO CLAIM IS MADE TO THE EXCLUSIVE RIGHT TO USE "SEO" APART FROM THE MARK AS SHOWN
SEO IMAGE
Registration Number 3180641
Disclaimer
NO CLAIM IS MADE TO THE EXCLUSIVE RIGHT TO USE "SEO" APART FROM THE MARK AS SHOWN
SEO SUPER REPORTER
Registration Number 3186487
Disclaimer
NO CLAIM IS MADE TO THE EXCLUSIVE RIGHT TO USE "SEO" APART FROM THE MARK AS SHOWN
SEO INFORMER (and design)
Registration Number 3233468
Disclaimer
NO CLAIM IS MADE TO THE EXCLUSIVE RIGHT TO USE "SEO" APART FROM THE MARK AS SHOWN
SEO KING
Registration Number 3246308
Disclaimer
NO CLAIM IS MADE TO THE EXCLUSIVE RIGHT TO USE "SEO" APART FROM THE MARK AS SHOWN
This one just issued, and contains a number of screwups:
100% ETHICAL SEO
Registration Number 3300009