Complex sentence: By the turn of the century there over a 1,000 families (and at least 500 students in the religious school) in KI, which had become one of the biggest (perhaps the biggest) synagogues in the nation.
Split into simpler sentences: By the turn of the century membership had increased to more than a 1,000 families and there were at least 500 students in the religious school. KI was now one of the biggest (perhaps "the" biggest) synagogues in the nation.