You can use the different Update Manager deployment models in different cases, depending on the size of your system.

You can use one of several common host-deployment models for Update Manager server:

All-in-one model
vCenter Server and Update Manager server are installed on one host and their database instances are on the same host. This model is most reliable when your system is relatively small.
Medium deployment model
vCenter Server and Update Manager server are installed on one host and their database instances are on two separate hosts. This model is recommended for medium deployments, with more than 300 virtual machines or 30 hosts.
Large deployment model
vCenter Server and Update Manager server run on different hosts, each with its dedicated database server. This model is recommended for large deployments when the datacenters contain more than 1,000 virtual machines or 100 hosts.