The idea is really good. The sfx fit well. I love the graphics, simple but efficient. The spawn is well implemented. The difficulty is well increasing.
There are only 2 downsides : your game need accurancy, but it's hard to move the stick 2 millimeters to aim -> better if you just have left/right inputs multiplied by a settable sensibility (check asteroid);
The second downside is the control of several ships, it's a bit frustrating to have 5 ships that all crash together -> make them repulse themselve, or anything that help us to keep away from them.
A music which grows/decreases in power with the ship count would be perfect.
Really well done !!