While, no exact number can be known, estimates are possible.
In a survey done with Reconstructionist, Conservative, and Reform rabbis, 33% ofno Conservative rabbis said they would not conductperform an interfaith marriageintermarriage, while 66%36% of Reform rabbis said they would refer the couple to another Rabbi, and 62% of Reconstructionist rabbis would.
In a population survey, 33% of American Jewish Families were "interfaith families", while another study says that currently, just over 50% of marriages are "interfaith marriages"
According to this article, It's hard for me to tell if 33% of intermarried Jewish families had a Jewish mother, or if 29% of Jewish woman, and 33% of Jewish men were intermarrying. The quote is..
Between 1980 and 2004, women, for the first time, were intermarrying in similar numbers to men (33% to 29%, according to the 2000-2001 National Jewish Population Survey).
Based on this information, About 18% of Reform Jews today would not be halachically Jewish. And 33% of Reform children born now, would not be halachically Jewish.