I don't know where to find industry standards that would help answer this question. But there might be another way to convince yourself - and your bosses - whether or not you really need more people. Usually, bosses don't understand the details of what has to be done in order to do what they want done (i.e., there are always problems that arise when attempting to do a task). Therefore, bosses tend to think that enough people are already available and that the problem is that they should be moreefficient. This is why managers often get the blame but not the required support.
Consider an automated task tracking system to help you clearly define who is supposed to do what and when. This system should include automated support for planning, including an estimated budget and start-finish schedule for doing each task. When the software adds up all the tasks for all of your people, and shows you the projected time-lines, you can make a more valid estimate of how many people you really need. This same system should also be used to monitor your entire organization's performance, providing valid support for making better future estimates, for performance reviews, salary reviews, awards, layoffs, etc. You might want to check http://www.adv-management.com for information about one system that should meet your needs at a reasonable price.