Number of papers is a notoriously poor measure of research output. For example, is one major paper worth less than 2-3 minor papers? 

Some people are really productive for various reasons. Sometimes people have been very lucky and their projects have worked out very quickly. Also, everyone develops as a researcher at different rates. As you get more experience you will be able to avoid many time consuming pitfalls and your publication rate (and quality) should increase. So maybe you're behind your peers and maybe you're not, it's impossible for us to say.

If your supervisor says your progress is good than I think you need to accept this at face value. If you're still concerned about it you should have a frank discussion about your concerns with your supervisor and/or another mentor in your field who can give you an unbiased view of your progress and prospects for an academic career.