The answer is 2, and it's quite a simple equation, really!
How To Work Out The Answer
First, we need to work out how many photographs Roger can fit in one album. We do this by multiplying the number of photos per page by the number of pages, so:
4 x 18 = 72
Then, we need to work out how many spare photographs Roger has left. We can do this by doing the math:
105 - 72 = 33
Or, we can just double 72 and see if the result is equal to, or greater than, 105. This is probably easier, as it's quite clear that 72 x 2 is going to be bigger than 105 (it's 144, in fact).
This means, to accommodate all his photographs, Roger will need 2 photo albums.
He won't fill the second one completely, though!