Hi Kat, thanks for reaching out.
What is your exact query? I just tried
select mag_auto_r from des_dr1.galaxies where mag_auto_r<99 and snr_r>10
and get 7890443 rows, whose histogram looks like this:
I've marked 17.7 with a vertical line. So, there are some galaxies brighter than that, but not many. See also Fig. 10 in the DES DR1 paper: https://ui.adsabs.harvard.edu/abs/2018ApJS..239...18A/abstract
Hope this helps. Let us know if we can help more.