I have been asked that question SO many times. And the answer...well, it depends. There are several factors that determine the reed strength. Obviously, the experience of the player, but's that not all there is to it. The strength of the reed will vary with the mouthpiece. In general, the larger the tip opening, the softer the reed should be. At the end of this article, I've included a comparison chart of a few frequently used student mouthpieces combined with a reed that is a good STARTING POINT. Please note that a "3" reed is not the best choice for every mouthpiece AND - not all "3's" are created equal. They differ from brand to brand and even within the same brand. (Example: A Vandoren Blue Box is a bit harder that a Vandoren V12.
Another factor that effects reed response is the ligature. In very simple terms, the more of the ligature that is touching the reed, the more resistant the reed becomes. A cloth ligature with give a bit "stuffier" sound and create more resistance for the player than a metal ligature with rails. But the cloth one is probably a better choice for a beginner because it also provides more control. I haven't included any info in the chart about ligatures. There are just too many variables!
Please don't interpret this chart as a "one size fits all" solution. Remember, this is a STARTING POINT! Experiment! If the sound is always stuffy and raspy, go to a softer strength. If the reed seems to be always closing up or the pitch is usually overly flat, try a harder reed. What works for one student might not work for another with the exact same mouthpiece and ligature. Don't be afraid to try a softer or harder reed. USE WHAT WORKS!
NOTE: As the embouchure develops, most students will need to move up a 1/2 strength
Marilyn Mattei is the author of For Clarinets Only. Visit her web site for more information.