This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Do you mean is there a problem with having an instance of each servlet for each user? Hard to say without knowing more, but I would write my servlet to be thread safe and have one instance per servlet no matter how many users come to the site.