That was fun! Good stuff.
You could simply make a section with 8 monkeys and when user hits a certain score hide the section with 4 and show the section with 8.
Some food for thought…
You have a lot of repetitive code though. The code for each monkey is almost exactly the same. 4 monkeys - 4 sets of almost identical code. 8 more and you’re up to 12 sets. Ideally you would have one set of monkey code and use it over again for making each monkey maybe throwing in some random differences in speed for each one. Set up this way you could create (or destroy - have you considered exploding monkeys?) as many monkeys as you wanted whenever you wanted. Grid would take care of the layout.
Maybe have just one click handler for the entire page that when user clicks you were able to detect 1) was a monkey clicked 2) if so, which monkey was clicked. Search for “event target” or “event delegation” to get an idea how that would work. Or maybe not. Possibly each monkey should have an attached click handler