Because DITA is an architecture, not just a DTD, you have to ask the question in terms of which infotype (concept, task, reference) , containing which domains. New specializations always increase the count, but the new content models typically have more restricted content models, which limits the selection choices that writers actually see in a validating XML editor.

The following count is based on the developerWorks-based dita13 toolkit, which predates the OASIS DITA 1.0 Standard.
The unspecialized topic dtd in DITA has 94 elements.
The 4 basic domains (software, programming, highlighting, UI) contribute 49 elements, which brings the count for domain-specialized topic dtd to 143 elements. Since domain specialization inherits across all dtds derived from topic, this is the base count affecting the rest of the per-dtd totals.
Concept.dtd adds 2 elements for its single-dtd total of 145 elements.
Task.dtd adds 25 elements for its single-dtd total of 168 elements.
Reference.dtd add 12 elements for its single-dtd total of 155 elements.
Ditabase.dtd adds 1 element but includes the sum of new elements in all the infotypes, having a grand total of 183 elements.