Actually none in Texas. All you need to teach algebra or anything as a public school teacher is a teaching certification that says you know algebra. Whether your degree in college relates is immaterial. Now having a degree or classes related to algebra won't hurt you in fact it would help you in securing a position as a teacher.
Take me for example I taught elementary special education and was transferred to third grade math and I had a degree not related to anything that would help me teach it or understand what I was doing. What I had was a certification that said I knew what I was doing and a unrelated degree.
In summary you don't need need a degree related to your teaching field, you need a certification and that is it. The degree in mathematics only helps get you hired and understand the material. Also, if you do get hired to teach mathematics algebra might not be the only course you teach. Remedial math and higher math could be in the cards so brush up on your understanding of those. Finally, take courses on how to teach as well if your in college. Many teachers are woefully under prepared for the realities of the classroom and the bureaucracy of education.