I'm afraid this one is not task for Forest Pack as it's scattering solution is based on probabilistic algorithms.
With all probability values normalized you'll not receive desired effect - because of it's randomization it'll be close but not exact, as there is Random Seed parameter influencing the result. Also the total amount of scattered objects is defined by it's Density settings not by exact number.
Anyway, by using Custom Edit mode under Tree Editor tab you can Add your models manually using Sequence. Attaching screenshot showing this option.
Can you please describe more in detail what are you trying to archive?