Complex sentence: John's Children were active for less than two years and were not very successful commercially, having released only six singles and one album, but they had a big influence on punk rock and are seen by some as the precursors of glam rock.
Split into simpler sentences: John's Children were active for less than two years and were not very successful commercially, having released only six singles and one album. But they had a big influence on punk rock and are seen by some as the precursors of glam rock.