How often do Master's programs accept applicants with online Bachelor's degrees?
Regarding question 2, the United States government does not track graduate admissions rates, but test preparation companies and academic communities may conduct surveys that give you a good sense. See the answer for Where to find grad school acceptance rates?  Some programs offering online undergraduate degrees might poll their students to learn the graduate admissions rates, but it might be hard to verify those numbers.

I would suggest that "students who have online degrees" is not the most helpful classification to use in determining your chances, because it is a large and varied group. The (U.S.) National Center for Education Statistics presents some data on how many students are taking online classes in American institutions. The takeaways are that online enrollment numbers are growing, that most students do not pursue 100% online programs, that different kinds of schools offer this (public, private non-profit, and private for-profit), and that the students who take online classes are more likely to be "non-traditional" (based on age and employment). Not all online programs are equal.

Instead, I would suggest you look carefully at the school offering the online degree. What is its reputation, is it accredited, how is it ranked in U.S. News &amp; World Report (or international counterparts), what job/grad admissions resources does the college offer to its current students, and what does the school website say about where its students end up? Also find out whether the people teaching classes have advanced degrees, and take a skeptical look at how much contact you will have with professors during your online study. (Talking with classmates, instructors, and professors helps you learn, and it gives professors the chance to understand your goals and provide advice and potentially later a recommendation.)

Good luck with your studies and decisions!