This isn't a simple a math question as much it is a "How long should my book be?" question.
If it's a simple math question, the answer is easily found by using the word count function in your word processor. Take the total word count, divide it by number of pages you've written and that's your individual word count per page. Then divide your goal by that number and that will give you an idea of how many pages you need.
If you're not a big fan of math, then you can eliminate the guess work by using standard manuscript format. In standard manuscript format, your word count becomes "about 250 words per page" which makes 350 pages about 87,500 words. If your goal is 100,000 words then you're going to be looking at 400 standard manuscript pages.
But what you're really looking for isn't a word count. What you're looking for is how much you need to write before your regular novel is considered an 'epic' novel. And there is no straightforward answer.
But one thing you can do is to look at previously published 'epic' series to get an idea of what readers expect when they want to read an 'epic'.
One of the most prominent epic fantasy series is A Song of Fire and Ice (also known as A Game of Thrones). The five books in that series (to date) average between 300-450,000 words each. That epic length is 3-4 times what you're setting your goal at.
So a story on the scale of Game of Thrones might be a tad much. But there is another prominent epic fantasy series that might be a little closer to what you're thinking.
At 450,000 words, the Lord of the Rings trilogy is almost certainly considered an epic fantasy tale (if not, the epic fantasy tale). Most people think of it as three books of about 150,000 words each but it's actually 6 books of about 75,000 words each. Despite the original plan of combining all 6 (plus one other, The Silmarillion) into one giant book it was instead collected into the three volumes that we're more familiar with.
Compared to Lord of the Rings, your 100,000-word, 5-book goal is certainly within the realm of 'epic' tales, albeit a little on the short side for the genre.
The word count for a page will vary depending on font size and type, margin size, and spacing elements (single/double space, blank lines, subheadings, graphics).
For a page with 1 inch margins, 12 point Times New Roman font, and minimal spacing elements, a good rule of thumb is 500 words for a single spaced page and 250 words for a double spaced page. Using this as an example, a 3-4 page double spaced paper is 750-1000 words, and a 7 page double spaced paper would be 1750 words.
Assignments often specify a research paper or essay length in terms of words, rather than pages - a paper of 750-1000 words or a paper of 1500-1750 words. This way a student's paper will still meet their instructor's length expectations, regardless of varying font size, margin size, or use spacing elements.
When viewing an electronic version of a student paper in MicroSoft Word, the exact word count can be easily determined. Some research assignments require students to include the word count of their paper.
Also, clarify with your instructor whether the words on the title page, abstract (if used), and reference list count toward the expected word/page count.