Does the number of Participants affect the quality of a conference?

For what exactly? What format etc?

If your hosting the conference yourself then it really depends on a few factors, If the quantity of participants puts strain on the CPU, Memory or Bandwidth, then yes the quality could degrade. If your using a service, go with what they say. They know what they can handle and should not be selling packages that will degrade with the number of users.

what conference software are you using , is it customized