Quantifying what you need vs. what you have
First you need to quantify what you need and compare it to what you have. You can do this in a table if you want to break it down in detail, such as by multiple locations or by labor categories. But comparisons are better made visually. One way to do this is with a pie chart:
The goal is to show that the gap is really small and you already have the vast majority of what you need.
The best way to prove that you already have the staff is to name names and explicitly identify them as existing hires that you commit to delivering if you are awarded the contract. The next best way is to show letters of commitment by the staff to join your firm upon award.
Quantifying that you can fill the gap
Once you've shown the gap, you still need to show that you can fill it. This is a similar issue. You want to show that you have access to far more staffing resources, either within your company or on your team, than you need to fill the gap.
Again, visually works best. You want to show that what you need is tiny compared to the number of relevant applicants who are directly within your reach. These might include the number of resumes you have received and track in a database. They could also include the resumes that your team members have in their databases. They could include other sources that you use for recruiting, such as estimates of the draw from placing advertisements.