I have 2 vertical indents growing in my thumb nails once it grows out another appears. Why?

Get it checked. Horizontal nail deformities (Beau's lines) can be from severe illness: heart attack, severe infection, surgery, or from chemotherapy. Vertical deformities (central ridges) can be from iron, folic acid or protein deficiency. Other vertical deformities (central canal) can be from severe arterial disease, malnutrition, or repetitive trauma. Your clinician can help you sort this out. Hope that helps.