I have a Z3 2000 and the chek engine light is on the code given is p0411

Secondary Air Injection System Incorrect Flow Detected ...that's the air pump that introduces air in to the exhaust stream before the cat to help burn off unburnt hydrocarbons........ http://www.obd-codes.com/p0411

I was told it may be a bad gas cap.... Is this a serious problem? I have just had two valves and hoses in this area repaired? How many are there?

What you may have heard a very common cause for the CEL (Check Engine Light) to come on is a bad or missing gas cap. And it's true. But that is part of the EVAP system, but you have a code. A specific code, not part of EVAP. Both are part of the emissions systems. though. There is a lot of information on that link I gave, I can only direct you there, not copy and paste the causes/solutions, it's copyrighted.

OBD aka DTC codes that begin with P04xx--are emissions....P0400 through P0499 are all emissions.... and P0440 through P0459 are EVAP, i.e. if it was the gas cap the code would be in there. It isn't. Now P0410 through P0459 (including yours) are Secondary Air Flow problems. http://www.obd-codes.com/trouble_codes/