Relativistic runaway electron avalanches (RREAs) occur when relativistic electrons undergo avalanche multiplication when driven by electric fields. The RRE avalanche has been studied extensively, but existing results typically assume a rudimentary source of seed relativistic electrons. Here we focus on the seeding process and simulate effective seeding efficiencies for various seed particle types, energies, and geometries. Including known results from cosmic ray physics, we calculate total seed particle distributions and their statistical fluctuations and use the seeding efficiency results to determine the total number of effective RREA seed particles at various points in the atmosphere. The results indicate that effective seed flux is quite large with only moderate statistical fluctuations.