Myself, I try to read at least 2 chapters a day. In the book I'm currently reading, chapters are about 10 pages, so I'm usually reading 4 or 5 a day. I think a number of chapters makes more sense than a number of pages, but chapter length can vary quite a lot, so I think it would help to be flexible with your goal. Be willing to change it on a book to book basis.

In my opinion - it's better to set a low goal - one that is attainable. No point beating yourself up for failing to meet unattainable goals you've set for yourself. Set a realistic goal, and if you read more than that on some days, great.