I use the whole alphabet and more:

• a, b, c, d for constants
• e is Euler‘s constant
• f, g, h for functions
• i, j, k, l for indices but can be used for natural or whole numbers
• m, n for natural numbers but can be used as indices
• o, p, q, r, unreserved although I sometimes use p and q in a rational equation like: p / q
• s, t function parameters
• u, v, w vectors
• x, y, z variables

Greek letters for angles or literally anything else.