Generating multiple tones

Hello. Are there synthesizers that can generate multiple tones simultaneously and allow you to modulate them individually on-the-fly? I would imagine that only one tone or sound can be produced at a time -- what kinds, if any, of exceptions are there to this one-at-a-time rule?

Thanks

Well most modern keyboards are multi-timbral ie. can play more than one sound at a time. When you talk about modulating those sounds individually what exactly are referring to? Tell me what it is you want to be able to do?