It's best to get a boat and sail around the coasts of black forest biomes. That said, it's not guaranteed. I've looked at a bunch of random generations on the valheim map site, and generally it seems like there's 4 closer to the centre, though more commonly in the centre ring, and 6 closer to the outer edge. You can try and find him without assistance, though if frustrated, you can always use https://valheim-map.world/ . If you don't want any major biome spoilers, you can turn off terrain so the map is covered by fog of war, and select the trader marker.