In the Standard Model (SM), there is no constrain for how many generations of quarks should or could have.
In a recent article “Three Generations in Minimally Extended Standard Models, http://arxiv.org/abs/1205.4483 (written by Paul Frampton, Chiu Man Ho and Thomas W. Kephart)”, it stated that all anomalies in the Standard Model can be canceled if there are three and only three generations. They can find infinitely many models that cancel all gauge anomalies, but all of these new models require three generations.
However, this new discovery was published in the book “Super Unified Theory” (ISBN 0-916713-02-4, Copyright # TX 1-323-231, Library of Congress Catalog Card Number 84-90325) long ago, and the brief discussion of this issue is also available in the article “48, the exact number for the number of elementary particles, http://prebabel.blogspot.com/2012/04/48-exact-number-for-number-of.html ”.
In fact, in Super Unified Theory (Axiomatic Physics), the three and only three generations are the absolute, way beyond as a requirement for cancelling the anomalies.