In string theory, the number of "gigantic" dimensions in the bulk depends on the specific formulation of the theory and the particular scenario considered. In most versions of string theory, the universe is assumed to have nine or ten macroscopic dimensions.
In the early formulations of string theory, it was thought that there were nine spatial dimensions and one time dimension, making a total of ten dimensions. However, as the theory evolved, it was discovered that certain versions of string theory require additional spatial dimensions to be consistent.
One popular version of string theory is called M-theory, which encompasses various earlier string theories and provides a unified framework. In M-theory, it is believed that there are eleven dimensions, with ten of them being compactified into tiny, curled-up dimensions.
The nature of the remaining "gigantic" dimensions in the bulk depends on the specific compactification scheme and the geometry of the compactified dimensions. Different compactification schemes can lead to different types of "gigantic" dimensions, such as flat or curved geometries.
It's important to note that the concept of "gigantic" dimensions in the bulk is a simplified analogy used to help understand the compactification process in string theory. In reality, the nature of these dimensions and their relationship to the observable macroscopic dimensions is a complex subject that requires sophisticated mathematical techniques and further research.