There are one or two "hot ticket" races in my area (Virginia) that fill up very quickly. For instance, there's a race called the Jefferson Cup in Charlottesville (an hour or so west of me). The 4 and 5 fields filled up this year to max capacity of ~75 riders within 2 hours of registration opening online, something like a month before the event. All depends on where you are, talk to some local racers and get some advice.