As already specified in the previous answers, the choice of K-Grid mesh should be taken upon verifying the convergence of the desired quantity. We usually start with the convergence of the total energy, but for other properties like optical spectra, for example, the converged grid with respect to the energy should not be enough, and a denser grid is usually required.
With respect to the question
"What will be the effect on band structure if I take very high values of K-points or low value of K-points?"
To the band structure calculation, the path in the Brillouin zone should be explicitly given. A denser grid leads to a more resolved band structure, however, the computational cost increases significantly with respect to a coarser grid. For a coarse grid, details of the band structure could not be properly resolved, however, the calculation time will be reduced.
The size of the primitive cell should also be taken into account. For large (super)cells fewer k-points are required since the Brillouin zone is decreased with increasing the cell.
Last but not least, for the Monkhost-Pack grid Quantum ESPRESSO allows for shifting the grid by setting
Kx Ky Kz 0 0 0 (non-shifted)
Kx Ky Kz 1 1 1 (shifted)
Depending on the symmetries of the structure, the shift moves the k-point mesh semilattice. The number of inequivalent points then decreases, resulting in a reduction in the total number of k-points. More information can be found in the Material Square blog.