The number of dimensions depends on the context:

• In everyday observable reality, there are 4 dimensions: three spatial dimensions (length, width, depth) and 1 time dimension, which together form the four-dimensional spacetime we experience.

• In theoretical physics, especially string theory, there are at least 10 dimensions: 4 familiar spacetime dimensions plus 6 additional spatial dimensions that are compactified or hidden from everyday perception.

• Some theories like M-theory propose 11 dimensions, incorporating supergravity.

• Mathematics deals with dimensions more abstractly, and can have spaces with arbitrary or infinite dimensions depending on the application.

So fundamentally, 4 are observed in our universe directly, but modern physics suggests more dimensions exist beyond human perception. The exact number depends on the theoretical framework applied.