Pages is a meaningless measure. A novel is typically around 90,000 to 120,000 words (but could be much more or much less). The size of the print and the dimensions of the book page determine how many pages.
I know people whose handwriting is the equivalent of 8-pt. type and others who write in 24 pt. There is no way we can make an estimate for you. We don’t know what you think is “medium.”
Count how many words are on one of your own typical handwritten pages. Then multiply that by the number of pages you have. That will give you an approximate word count.
If you’re looking for an estimate in manuscript pages rather than finished, printed book pages, remember that standard manuscript format is double-spaced (that is, has a line of space between every two lines of text), so there would normally be about 250 words on a “typed” manuscript page in a standard 12-point font, double-spaced.
There is nothing wrong with writing a draft by hand. I often start in a notebook; Nabokov wrote his novels on index cards. You’ll need a word-processed electronic manuscript when it comes to serious submissions to a publisher, though, and that means a lot of transcribing.
I found a set of stats for the Harry Potter novels online in several places but didn’t find an authoritative source. They’re probably close enough to serve as an illustration. Here’s one:
Philosopher’s Stone- 76,944 words
Chamber of Secrets – 85,141 words
Prisoner of Azkaban – 107,253 words
Goblet of Fire – 190,637 words
Order of the Phoenix – 257,045 words
Half-Blood Prince – 168,923 words
Deathly Hallows – 204,796 words