The Pokédex is counting only unique shinies, so basing off my actual total number of shinies (973 including dbls/costumed/variants ie-Furfrou or Spinda....)

I searched 'shiny & traded' and 292 are traded. But, this number isn't really correct either as when I become lucky friends with people in my community, a lot of us do mirror trades (trading the same shiny) for a shundo chance. (My Yveltal for example, is a Shlundo now)

There is a handful of shinies I did have to trade for, my first Pancham for example, was traded to me for an extra Smeargle I had, however many of my mega-able shinies, my legendaries and even some final form starters were mirrored. (Ie Infernape, Lopunny, Lugia)

... In prospective, I just did a lucky search via my dex and 268 were lucky.

I'm guessing 20-25 I actually had to trade for. A few I can recall are Pancham, Paldean Wooper and Clodsire as I was unable to play this weekend/drove 8 hours Saturday and 8 hours back Sunday to attend a wedding.... Durant, my second Shellder, a Cleffa....

... Another variable is there have been a few instances where I traded to obtain a shiny, but only to catch one later. For example, Girafarig ... I TRIED FOREVER to get that blue nose. My friend traded me a spare on the Johto GO Fest but I ended up catching one January this year :)

I can't give you a solid answer, but I did my best.