UAC48, it sounds like you are trying to write a traffic generator to produce traffic at a specified rate.
In theory I believe the "right thing" is an exponential distribution for the arrival intervals. See
Poisson process - Wikipedia, the free encyclopedia for more details.
However if you plug in an arrival rate into this process, it will only give you that arrival rate on average; if you specify 10/20 then on average you will get 10 aircraft in 20 minutes, but in any given 20 minute slot you might get more or you might get less. If you want to have a 20 minute exercise where exactly 10 aircraft arrive then generate 10 random numbers in the range 0-20 and use those for the arrival times.
Is that what you wanted to know?