It does not matter how many Americans are for open borders and how many are against.
First, this term is vaguely defined. Hence, disagreements are meaningless.
Second, most Americans are deeply misinformed on politics; immigration in particular. Hence, it is irrelevant how many deceived Americans agree of something that makes no sense to begin with.
For those interested in understanding the core of our immigration mess: https://medium.com/@hak1010/troubleshooting-immigration-893c7d2d9f