As per my understanding you want to save 80 images in a .mat file and you are actually using a video to take snapshots of frames. The getsnapShot function returns just one frame, if you want to capture multiple images from a video you can use the VideoReader function to capture multiple frames and save it. Here is an example to do the same.
v = VideoReader('xylophone.mp4'); % reading video
frame = read(v,[1 80]); % Capturing first 80 frames and saving in 'frame'
save('images.mat',"frame"); % creating the MAT file in current directory
imshow(frame(:,:,:,15)) % Displaying 15th frame from 'frame' variable