When a problem is too complex to solve, then break it down into SMALLER parts, that you can handle.
You want to know how many rows in your array have the property that all elements are greater than or equal to 50. So, first, can you determine which elements are at least 50? (I hope so.)
What does that do? (Hint: it creates a logical array, that is true when the element is greater than or equal to 50.) A logical array is effectively just an array of 0 or 1 elements.
Next, what does the all function do? (READ THE HELP!) If you want to know if all of the elements in a row are greater than 50, what does this do?
What shape is that result? (TRY IT.)
Finally, how many of those rows have the desired property?
Break a too big problem down into sub-problems, that hopefully lead in the direction you need to go. When you get stuck, look in MATLAB for tools that will help you to solve the problem you are currently stuck on.