If you bet USA racing. Charles Town run almost all year round four days per week.
They have a maximum field of 10 but can list up to 14 runners in some races.
Once these markets are live then deductions will be made for withdrawn runners BUT its unusual for the races at Charles town to go below 125% on the back side until at least 15 mins to the off. At this time of course all reserves and scratches will be known ( except very late vet scratches).
Delta Downs do the same,but don't race all year round.