I just checked and the used O2 concentrator I am using in my shop here is only 5L, but I find in adequate for my trade repair shop, using small propane bottles and the Small torch.
I was trained on the larger Hoke torches, and I scoffed at the tiny torch when I first encountered one at a shop I went to work for, but it is plenty for 99% of what I have encountered doing full service repair work.
Last week I had to anneal a heavy 6mm Tiffany Platinum band, and today I had to melt roughly 1 oz of Gold to roll.
This system handled both jobs well.
One of my Hokes on large O2 and propane tanks might have got the job done more quickly, maybe, but this got both jobs done satisfactorily.