Most vises have four bolt holes, so you will need four bolts to mount the vise. The size of the bolts will depend on the size of the vise and the size of the bolt holes. For example, a small vise with small bolt holes may use 1/4-inch bolts, while a large vise with large bolt holes may use 3/8-inch bolts.
It is also important to consider the length of the bolts when mounting a vise. The bolts should be long enough to reach the other side of the vise and be securely fastened, but not so long that they protrude out the other side and could be damaged.