See my input below:
1. Given these requirements, what would be a good number of GCPs to use (how do you determine that figure?)
For an area of similar size (that was flat) we noticed that adding more than 5 to 8 Ground Control Points (GCPs) did not improve the accuracy much more, i.e. the time investment of adding GCPs would not result in a large improvement of accuracy. As your area is hilly, I would add a few more GCPs to cover low and high areas. Make sure that the GCPs are evenly spread and are inside the area of interest so that many images see the GCPs. This makes sure that they have a better impact on the project. If I were you I would measure a few more GCPs that can be used as checkpoints to verify the accuracy of the results.
I can recommend this article from the Knowledge Base: https://support.pix4d.com/hc/en-us/articles/202557489
2. What would be a good minimum sidelap / overlap?
The general recommendations for overlap are minimum 75% frontal and 60% side overlap. However, if your area is desert like, I would increase this minimum to 85% frontal and 75% side overlap. There is more information on flight plans in this article: https://support.pix4d.com/hc/en-us/articles/202557459#label1
I would try to reduce the height differences during the flight and in any case make sure that the Ground Sampling Distance (GSD) at one place is not more than twice the GSD at another place. Otherwise, there could be issues during the matching step of the initial processing. More on this in these two articles:
3. Also wondering could two people using two identical drones, such as a DJI Phantom 4, could be used to save time or would possible variations in the altitude measurements between the two drones possibly introduce large errors in the resulting work product?
It is possible to use two drones for a single project. As long as you have GCPs in your project there should not be an issue due to the image geolocation. Make sure that you have enough overlap between the flights of both drones and that there are not too much differences in the environment (e.g. shadows,…). This could result in multiple layers in the point cloud or other effects which would require additional manual work. For such a large area it may be of advantage to use a fixed wing drone, which has a longer flight time.