Billboard magazine began tracking the popularity of country music songs in 1944 and it is widely considered to be the standard music popularity chart in the United States.Sonny James would continue to hold the standard with 16 with his albums "Need You" ,"Here Comes Honey Again"
and "Only Love Can Break a Heart"
In 1957, this young artist and his song, "Young Love", topped both the pop and country charts. This honor has not been achieved since and was the first of many hits and a string of recognized hits that
ended with his final Number One, “Is It Wrong (For Loving You)” in the mid 70's.
Sonny is not a country artist, thus it does not matter where he places on the R&B charts. He is not the only country artist to reach number 1 with an R&B song because he is not a country artist. He cannot be the only one to reach it if he is not one in the first place.
Sonny may have had a hit as a pop artist before it was popular, but today many country artists cross over to other charts. The country music when Sonny was big is different than what we hear today. For Sonny to have a R&B hit in his time was an accomplishment. Today, it is common and I would expect to see many cross over hits.