Complex sentence: Though it is impossible to determine how many garage bands were active in the 1960s, according to Mark Nobles, it is estimated that over 180,000 bands formed in the United States, among which several thousand made records.
Split into simpler sentences: Though it is impossible to determine how many garage bands were active in the 1960s, their numbers were extensive. According to Mark Nobles, it is estimated that over 180,000 bands formed in the United States, amongst which several thousand made records.