For CCNP, you can take couple of these to practice all the basic layer 2 features. If you want advanced features like RSTP, MSTP, dot1q tunnel etc. you might need atleast 4-5 of these switches. I would say it depends on what topology you will be setting up for practice. You might also need a L3 switch switch 3550/3560/3750 for L3 features.
For CCIE, you need atleast 4 L3 capable switches two of these would be 3550 and rest can be either 3560 or 3750 switches.
-amit singh