[QUOTE=JustTheTicket;7892030]
about a million polo shirts[/QUOTE]
At least!
But yes, it depends on what you’ll be doing.
Several pair of mid-high quality breeches that can stand up to being worn all day. Even if you only ride 1x per day, you’ll likely end up living in them since there’s just never time to change.
A couple good pairs of gloves, spurs, and crops. They always go missing (and gloves get gross and need to dry out).
A good pair of work boots that are quick and easy to get in and out of when you ride. Don’t skimp. You’ll be on your feet all the time. Similarly, if your tall boots are not comfortable to be in for 14 hours, consider upgrading. Insoles help too.
If you’re somewhere cold, get some nice, warm, neutral colored clothing.
Some will want you to have your own saddle, so do consider that. (And others will require you use theirs. It’s personal preference of the trainer.)
A good bag, especially if you’re going to be grooming on the road. But I also like to have my things contained, and if you’re not bringing your own horse, you may not have your own designated “spot” in the barn. When I was in California, I really loved my well insulated cooler/bag. I kept cold drinks and tupperwares of snacks. Again, there’s never enough time. And when I get really hot/cold/busy, I don’t get hungry, so having quick snacks that over the course of the day make up a meal was really nice. If you’re somewhere cold, a good thermos for coffee/hot chocolate/tea to help warm you up from the inside out.
Heating pads (like those rice bags you microwave), rollers to stretch your muscles, and maybe a yoga pad to stretch when you get home. You have to take care of your body.