Product Design team- ideal ratio between senior and junior designers

Our team has grown organically in the past couple of years. We have 2 senior designers, 1 mid level designer and 4 junior designers. Have been finding it difficult to find time for each of them. Senior designers who are mentoring junior designers are finding it difficult to complete their work. What’s the idea ratio between senior and junior designers in a product design team? Design managers, are you also facing similar issues? How are you working on it? Please share your thoughts and feedback.