As with everything else in life, "it depends." 

We receive this question with some frequency. Typically, a portfolio manager wonders if activist investing requires 10% or 20% or more of the outstanding shares, which it almost always does not. So, one general answer is also, "less than one might think." 

In activist investing it does depend - on what the investor wants to accomplish.  

We provide a discussion and a brief guide in a current blog post.

Previous blog posts

Majority Voting Instead of No Poison Pill or Declassified BoDs? Really? (February 20, 2014) 

Investors' Only Two-and-a-Half "Key Governance Priorities for 2014" (February 18, 2014)

Apple's Cash, an Accounting (February 11, 2014)

Herbalife May Be a Great Trade, But It's Not an Activist Investment (February 4, 2014)

Why Does Carl Icahn Bother With Nonbinding Proposals? (January 28, 2014)