In answer to your question, experimental data up to now have given us a number of particles that can be incorporated in the general framework of String Theory, which is a theory that also incorporates the quantization of Gravity.
String Theory does not allow an infinite number of particles if you think of them as being nested like russian dolls, more and more composite. It allows for a mathematical description of the Standard Model which incorporates particles we have seen, also predicts new particles but not in the nested form you are suggesting.
There are many types of infinities: The energy levels of a potential well are infinite, but at some point the distinction of the levels becomes trivial energetically.
At the moment we believe that quarks and leptons are the real "atoms" , i.e. non composite, components of nature.
If these experimental facts are contradicted by new data, it will be a big revolution in theory, including string theory which is aiming at being the theory of everything.