MATLAB: How to ensure that the number of matches between 2 images is equal to the number given

Using the matching features from here:

I1 = rgb2gray(imread('viprectification_deskLeft.png'));

I2 = rgb2gray(imread('viprectification_deskRight.png'));

points1 = detectHarrisFeatures(I1);

points2 = detectHarrisFeatures(I2);

[features1,valid_points1] = extractFeatures(I1,points1);

[features2,valid_points2] = extractFeatures(I2,points2);

indexPairs = matchFeatures(features1,features2);

matchedPoints1 = valid_points1(indexPairs(:,1),:);

matchedPoints2 = valid_points2(indexPairs(:,2),:);

figure; showMatchedFeatures(I1,I2,matchedPoints1,matchedPoints2);

How do I ensure that the number of matches found is equal to a number given e.g. 170?

If not, then how can I ensure that for every point in valid_points1 there is a corresponding point in valid_points2.

So if valid_points1 is 170 points in total, then indexPairs should also be 170 points.