Difficult question to answer easily.
There are basically 3 leagues.
USRA - US Racing League
UKRA - UK Racing League
NHC - National Hunt Racing League
You can choose one league as your primary league. This will be your main focus. Starting numbers for your primary league are 25 horses increasing to 36 as the season progresses if I remember correctly. You then will have the option to race in the other two leagues which will be classed as your secondary leagues. Starting numbers for these leagues are 12 horses to start increasing to 18.
You dont have to enter all 3 leagues. You can enter one, two or all three. That is your decision.
Horses you upload are your choice. You should study the schedule thoroughly and aim/target races. I would say that at least 2 (ideally 3 of most distance horses are needed)
There are upload windows every 4 weeks where you can upload new horses. You also have the option to retire horses in these windows to make more room. Maximum retirements throughout the season are 6 for your primary and 3 for your secondary stables. You also have the option to transfer horses between your stables. I think the limit is 5 transfers only per season.
The numbers I mention may not be spot on but they are close enough to give you a heads up.
If you have any more questions, ask away.
The SOPL (Starters Orders Pro League) International President Colm O'Dea is putting together a guide which should answer all your questions. This will hopefully be available to download shortly.
Jay