The most shards they will ever bring back is 60, which you can reach with just a level 10-15-ish retainer with level 10-15-ish gear. I use my three alts' retainers to farm shards for me, and I certainly can't be arsed to level and gear all 12 of them up, so I just left them in level 10-15 gear and it was enough to hit the 60 shards/hour mark.