When you design a replication system, you decide how many Replication Servers will be installed, which databases they will manage, and on which machine they will execute.
A Replication Server can manage one or more databases. One Replication Server is adequate for some replication systems. Other systems require a Replication Server at each geographic site to manage all of the databases at that site. Still others require multiple Replication Servers at each site to handle many databases or heavy transaction volumes.
When you upgrade Replication Servers, you can phase in the upgrades and install newer versions of Replication Servers initially only at certain sites. Consider the timing of such upgrades with care, and assess the need at each site for the new Replication Server features.