The same train of thought needs to be deployed when you are thinking about how many illustrations to include. It will depend on your goal, BUT the extra elements to consider are the age your book is written for and what the story needs.
The age range of your book is important.
The younger the audience of your book, the more important illustrations will be to hold their attention. In order for your book to be successful, more illustrations will be advised for your book. If you are aiming at a slightly older demographic, then the volume of illustrations can be much less depending on your genre of course.
What does the story need?
Primarily this decision will come down to what the story requires. You want readers to enjoy the story, and you want to do it justice. Which characters and scenes need to be illustrated to make your book the very best it can be? This question will help you make those difficult decisions when weighing up the number of illustrations compared to the cost.