Hi,
VMware recommand SQL Express for small environnement, the limit is for 5 Hosts and 50 Guest. If you have a linked vcenter environnement you will have a second database for this vcenter. So this vcenter will able to have 5 hosts and 50 guest too.
But you first database will not increase the limit, you still have the same.
You can have more information here : VMware KB: Installing vCenter Server 5.0 best practices
Microsoft SQL Server 2008 R2 Express is intended for use with small deployments of up to 5 hosts and/or 50 virtual machines.
It is for vsphere 5 but the limitation is the same on v4.
Julien.