Complex sentence: The most successful Argentine club on the international stage is Boca Juniors, they have won a total of 18 officially recognised international tournaments is a world record they share with A.C. Milan of Italy.
Split into simpler sentences: The most successful Argentine club on the international stage is Boca Juniors. They have won a total of 18 officially recognised international tournaments, a world record they share with A.C. Milan of Italy.