What filler episode is yahiko named catcher of a thousand in rurouni kenshin?

There is no episode of Rurouni Kenshin where Yahiko is called "Catcher of a Thousand". That title is not used for him in the anime or manga.

It's possible you're thinking of a different character or mistaking a fan-made title for an official one.

It's worth noting that Yahiko's skill in swordsmanship is often praised, and he eventually becomes a skilled swordsman himself. He's known for his speed and agility, which could be why you might associate him with a title like "Catcher of a Thousand."