"Seems too broad a job for one person"
Some folks can walk and chew gum at the same time:
"Bioengineering is the discipline of engineering that applies principles and methods from engineering, the life sciences, and the medical sciences to understand, define, and solve problems in medicine, physiology, and biology."
I'd wager there are a few that can walk, chew gum,plan a meal and mail a few letters - at the same time.
You could be right. And maybe a Bruce Lee could really defeat 50 trained attackers all by himself, with 50 wellplaced Kung-Fu kicks. It could happen.
How-ever: Your theory requires not just high-level competence in microbiology and true expertise in materials science and engineering. It also needs a mastermind - a plotting, planning, strategic risktaker. That is an unusual psycological makeup, and one not very often found in engineers and scientists.
My guess - 3 true insiders, maybe a few more at the periphery. I could be wrong.
This assumes that the mailers had the weaponisation secret and that they weaponised the anthrax themselves.
Another option: May-be the weaponised anthrax was stolen and mailed, after having been made by somebody else in some clandestine govt project. This does not require so many people now. Arguments against this: 1. That was a lot of weaponised anthrax to have stolen. 2. Whoever stole it, knowing they had a limited amount, probably would have chosen a better distribution method, or perhaps a multi-forked distribution with several vehicles.