One SPS should suffice to secure more applications. SPS acts like a Gateway unlike the WebAgent. WebAgent is deployed on the Application WebServer, however SPS sits like a gateway in front of the Application component. You could have multiple SPS instances running on one single server. However it is all about architecturally what is needed to be achieved and how secure / tighter the integration needs to be derives the solution.
Getting back to basics, One SPS should suffice. It all depends on how the SPS in combination with ProxyRules and Virtual Hosts are configured.
Take a look at this thread and suggest if it helps.
Secure Proxy Server 12.52 - Virtual Host Configurations
You should have Virtual Hosts defined for abc.com and dce.com. You could further improvise by having own WebAgent.conf for Virtual Host abc.com and dce.com. Using own WebAgent / Virtual Hosts provides you the flexibilty of defining different ACO objects, which could then mapped to different Agent Object and therefore realm definition would remain unique.
Let know if these pointers assist you in moving forward.