Guidance on number of accountanting positions there should be in a company?

We are currently considering our internal organisational structure, and there is a question over whether the number of accountants we have is appropriate. The company is an oil and gas co in western Africa, which is involved in exploration, production and distribution. At this stage we have successfully drilled wells and are building a gas distribution network. We already have some customers online (so we have invoicing and accounts receivable requirements) and we are also operating a process plant and continuing construction of the pipeline network. There are also some JV-style agreements in place which should be considered. Our head office is UK-based and is listed and audited so there are additional reporting requirements in this respect. Although we currently operate with only 100-125 staff and contract workers (which doesn't include the gas process plant, where operations and staffing are nearly entirely managed by another company), you can see that from a financial point of view there are lots of things going on. 

I am interested to hear how people think the accounting and finance function should be structured in this type of organisation, and even better, whether you have links to some kind of guidance on how to determine correct staffing levels. I can do this myself based on the various activities that need to be performed within this department, but I would like to add weight to my argument with references to external sources.