Scheme that encrypt with $n$ pk that any sk of the pks can decrypt? Suppose there are $n$ users each with public/private key pair $(pk_i,sk_i)$ $i=1,\cdots,n$. Is there any scheme that I can encrypt $m$ using the set of public keys $(pk_1,\cdots,pk_n)$, and can decrypt it using any $sk_i$?
That is $C=Enc_{(pk_1,\cdots,pk_n)}(m)$ and $m=Dec_{any\ sk_i}(C)$.

This is meaningful when these $n$ users are just a small portion of a group of $N$ users. A bit like "ring encryption" (opposite to ring signature). Thank you!
This is in no way practical, but it's relatively easy to describe a scheme.

If the public key contains (or allow easy derivation of) a unique prime (just has to be unique among the recipients - If every recipient knows the public key of any other recipient, and public keys can be ordered, we can use the first $n$ primes.), say $r_i$, you can use:
$$
C=\prod_{i=1}^n r_i^{\operatorname{enc}_{pk_i}(m)}
$$
(where $\operatorname{enc}$ is the function for encrypting to one recipient).

Then for decryption you "just" need to determine factor $C$ and determine the highest power of $r_i$ that divides it, and put the exponent into the function for decryption to one recipient.

I haven't tried to prove that it isn't easier to break this than to break than to break the scheme it's based on. I can't see why it would be though, and given how impractical this is, I don't think it matters much.