The amount of memory would be the variable to keep in mind. By default each mobile client is configured to use 256 MB of memory, so 256 MB X 14 clients = 3.5 GB of memory. Also, keep in mind that you may need to adjust the amount of memory used by mobile clients depending on project size and complexity. Then, take into account if you are using 32 bit or 64 bit Ignition. 64 bit Ignition will use 2 GB of memory by default and can be configured to use more. SQL server will also need a chunk of memory as well if it resides on the same machine.
Based on your >=8 GB, I would say that 8 GB could get you where you want to go, but its going to eat up almost all 8 GB. 16 GB would get you where you need to go and give you room for growth. 32 GB would be the next up. Considering how cheap memory has become, and most DDR3 motherboards support 32 GB of memory, I would go with the 32 GB. Its still up to your budget, but 16 GB would be more than enough as well.