There are 25 Bonus Boards, and they are shown as Influence Boards on your map. I recommend getting most of the other achievements first as you will discover many of them along the way. You can click on this picture to make it bigger
Here is a video by jidenshaotoko showing the how to get to the board at each location, since one is under a bridge and another you have to jump to get.
Be sure to give them a like if you find their video helpful.