The wisdom behind tying them is so that they do not come undone and separate.
As for the number of ties, whatever is needed should be used. It is known that the minimum needed is two ties: one at the head and one at the feet. Two or three ties may also be needed in the middle. As for the claim that there must be seven ties, I know of no basis for it.
“They are untied in the grave. If he is shrouded in a shirt, a waist-wrapper, and a wrapping, that is permissible. A woman is shrouded in five garments: a waist-wrapper, a head-covering, a shirt, and two wrappings.”
His statement, “They are untied in the grave,” is supported in al-Rawd by a report from Ibn Mas‘ud, may Allah be pleased with him, who said: “When you place the deceased in the grave, untie the knots.” (1)
This is because the deceased swells in the grave, and if he is bound by these ties, they will tear him.
If it were assumed that they forgot to untie them, then remembered shortly afterwards, the grave should be opened so that these ties may be untied.
Al-Rawd states: “Making holes in the wrappings is disliked,” because that damages them.
If someone were to ask, “If you make holes in them, would they not fail to conceal him?”
We would say: No; they would still conceal him. For example, one may make a hole in the upper wrapping, then make a hole in the one beneath it on another side, not opposite the hole in the upper wrapping; and the third likewise.
The author of al-Rawd mentioned this because some scholars said that, if grave-robbers are feared, the wrappings should be torn, because there used to be