According to the theory of string theory, strings are indeed postulated to exist in higher-dimensional spacetime. However, it is important to clarify that string theory suggests the existence of extra dimensions beyond the familiar three dimensions of space and one dimension of time, but it does not specifically state that these extra dimensions must be precisely 11 in number.
In the early formulations of string theory, it was proposed that the theory required ten dimensions. This is known as the ten-dimensional superstring theory. Later, it was discovered that certain formulations of the theory, such as M-theory, incorporate eleven dimensions. However, the exact number of dimensions and their nature remain open questions in string theory, and different formulations of the theory propose different numbers of dimensions.
These additional dimensions are typically compactified or curled up at extremely small scales, making them inaccessible to our current observations. The strings in string theory are one-dimensional objects that vibrate in these higher-dimensional spacetimes. The specific configuration and behavior of these strings give rise to the diverse particles and forces observed in the universe.
It's important to note that string theory is a complex and highly mathematical framework that is still actively researched and developed. While it offers a promising approach to unifying quantum mechanics and general relativity, it has yet to be experimentally confirmed. Our understanding of the precise nature and properties of these extra dimensions is still an area of ongoing exploration and investigation in theoretical physics.