In bosonic string theory, which is a simplified version of string theory, the number of spacetime dimensions is typically 26. However, it's important to note that this theory does not incorporate fermions (particles with half-integer spin, such as electrons and quarks) and does not describe our observed universe accurately.
In bosonic string theory, the spacetime dimensions are treated differently from the internal dimensions, which are compactified or curled up dimensions that are not directly observable at large scales. The 26 dimensions in bosonic string theory consist of 25 spatial dimensions and one time dimension. These dimensions play a crucial role in the formulation of the theory and the vibrational modes of the string.
However, it is worth mentioning that the more widely accepted version of string theory, known as superstring theory, incorporates supersymmetry and requires ten spacetime dimensions (nine spatial dimensions and one time dimension) for consistency. Superstring theory offers a more promising framework for describing the fundamental nature of our universe.
It's important to keep in mind that string theory is a complex and highly theoretical framework that is still an active area of research. Many of its aspects are still being explored, and there is no definitive experimental evidence supporting it at this time.