I have 14 RX550 and they are 12.5 to 12.8 each on the asus B250 mining expert.
can tweak some higher but thats the OC that keeps them all stable.
mem clock at 2100
and core clock at 1150
also used Polaris Bios Editor, I tried with the pro version but it just caused constant BSOD but free version did the timings for me then I slowly tweaked the cards with OverdriveNTool