Problem:

I'm currently working on a synthetic biology project which involves working with lots of different parts. I would ultimately like to integrate these genes by transforming a single plasmid. I've heard (casually) that 9-10 foreign genes on a transformed plasmid is often the practical upper limit for these types of experiments, without a real explanation other than "beyond that, it just doesn't work".

Required:

Question: Is there a practical upper limit to size of amount of genes in a transformed plasmid, assuming all need to be expressible?

Question: What causes this upper limit?Please explain in detail.