What percentage of ethnic Jews believe in the G-d of the Torah?
In my experience, it is well under half.
There’s no way to know. However you will see estimates of the US Jewish population ranging from 5.0 million to 8 million. The difference being how the questions are asked, and who the questioner thinks is a Jew since Jews don’t get to decide that. The lower numbers, 5-6 million, usually composed of individuals that answer a question similar to What is your religion? Jewish. Common sense. Then some surveys delve into followup questions. Was your mother Jewish? A yes would be a legitimate Jew, even if non religious. Your father, or grandparents. An interesting distinction, to my knowledge used only by the Reich and surveys. A yes answer adds another Jew to the survey, irrespective of actual religion practiced. I’ve seen some surveys which will throw in questions about Israel, and Christians displaying a high degree of interest may be labeled as Jews. So when looking at survey numbers, if they’re high, a quarter to third may not really be Jews. Then there’s the fact that outside of urban areas, there’s no way to survey Jews, there just aren’t enough. Even in exit polling, most statistics on “Jewish” voting are statistically insignificant.
I think it's about 10% of American Jews are Orthodox. However, they are much younger than the Reform or Conservative Jews and are having large families. Thus, their numbers are increasing.
I am excited that Sunday, my neighbors have invited me to their (Orthodox) son's upsherin, the celebration of his having reached the age of 3. I wonder what a 3 year old boy would like as a birthday present...