On account of the fact that Jewish law required 120 men to establish a community with its own council,1 some suggest that the number is symbolic and serves to say that there were sufficient believers for them to function by themselves. Luke himself states that there were about 120, suggesting that he has a literal (and not figurative) number in mind. He is simply giving an estimate as to how many people were present.
15 In those days Peter stood up among the brothers (the company of persons was in all about 120) and said,