I'm learning to jump and I heard that you should count the strides, but how do I count strides ?

I don't jump yet so I don't know if this will help, but when doing canter work over poles I basically focused on the horses movement and got a feel for how much ground they cover per stride. Once you can guess that a little then maybe try counting down from 3 before the jump, once you have that back to 4, or 5, or as many as you can to improve accuracy
If you're just learning, don't worry about it. Seriously. You have enough to think about (leg position, seat, release, control, balance, connection, breathing, etc) without counting. Once all those other things become muscle memory, then worry about counting. And even then, I use it much more for exercises than real life. Such as, if I set up a 6 stride, can I make my horse do it in 7? How about 8? Ok now 5 w/o him getting too flat & pulling rails. But right now, focus on those other things. Your horse will thank you as he cares much more about how you feel on his back /in his face than if you can count
Put up one jump, can be as big or small as you want but start of small, then put up another one probably about 5strides away - walk this out on foot when you're setting up jumps, and ride over these two jumps in a straight line. When you go over the first and you feel your horses feet hit the ground say out loud "land", then ever canter stride (3 beats) say "1", "2", "3", "4", "5," as you could alloud the strides and after the 5th stride your horse should jump the next jump. It's all about how many canter strides are between jumps. Practice this over different strides eg 2 or 1 or 3 or 7, also try on curved lines etc but count out loud, it will help
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