I think an average person can fall in love before marriage as many number of times as he/she feels like and this figure can differ from person to person...
As regards momentary attitude, there is no limit. Love begins at teenage. Teenage love is true love. First serious love will be at teenage. When you are adult, there is no love as such but only momentry physical attraction towards any well built and pretty woman. However, a man may on average fall in love with four women in entire life.