Security of the server's private key In some papers, 
server generates keys for users as follows:
$$sk_i=msk+H(ID_i)v_i\ (mod \ p)$$ 
where $msk=$ the secret key of server

$v_i$ is randomly selected from $Z_p$ for i-th user , $p$ is a prime number

$ID_i=$ the identity of i-th user

$sk_i=$ the secret key for i-thuser

$H$ is the hash function

If there is a user who knows more than one secret key, does that key generation scheme prevent to reveal the value of $msk$ from users.

How to prove the security?

I have forgotten paper references. Sorry for that.
Not really worked with these kinds of schemes, but here's my viewpoint.

Write $w_i=H(ID_i)v_i$. The value $H(ID_i)$ is some fixed public value, so we can kind of forget about it. That is, if $v_i$ is uniformly random, then so is $w_i$.

Now the key is generated as $sk_i=msk+w_i\pmod{p}$. Since $w_i$ is uniformly random, this is simply a generalisation of the one-time pad. In other words, $sk_i$ reveals no information whatsoever about $msk$. Therefore however many times you repeat this, the user will still learn nothing about $msk$.