There are several ways to scale up. I won't give you a complete overview of all possibilities here but give you a few pointers instead.
A single Sesame native store scales to about 100-150 million triples on typical hardware. Beyond that, you can either use a third-party Sesame-compatible store such as USeekM, Bigdata, CumulusRDF or OWLIM (which scales well into the billions of triples), or you can use Sesame's own Federation SAIL. The federation members can be any combination of Sesame-compatible stores, including native stores running locally or remote stores accessible over HTTP.
The Federation SAIL distributes write operations using a simple size-dependent sharding algorithm, trying to distribute data over all members equally. Queries are of course automatically distributed and results re-integrated.