According to the theory of general relativity, which describes the gravitational interaction and the structure of spacetime, the universe is a four-dimensional spacetime. It consists of three dimensions of space (length, width, and height) and one dimension of time. This is commonly referred to as "3+1" dimensions.
However, various theories beyond general relativity, such as string theory and certain approaches in quantum gravity, propose the existence of additional spatial dimensions. These theories suggest that there may be more than three spatial dimensions, but these extra dimensions are not directly observable in our everyday experiences because they are thought to be compactified or curled up at extremely small scales.
In string theory, for example, which is a theoretical framework that attempts to unify quantum mechanics and general relativity, the mathematics of the theory requires the existence of extra spatial dimensions. Different versions of string theory propose various numbers of dimensions, such as ten or eleven dimensions. However, it is important to note that these extra dimensions are not yet experimentally confirmed, and their existence is still a subject of ongoing research and investigation.
In terms of how many dimensions are needed to describe spacetime completely, the answer depends on the theoretical framework being used. General relativity successfully describes our observed universe using four dimensions. However, if one wishes to incorporate certain quantum mechanical aspects or explore unification with other fundamental forces, additional dimensions may be required in certain theoretical frameworks. The exact nature and number of these dimensions are active areas of research in theoretical physics.