I was able to perform an inner join using 28 views in my environment, it worked fine for me without any errors. In this situation, I would perform the following checks in order to get more information.
* Check the server logs in the data source side
* Check the Execution Trace under the Query Results tab of the join view
* Check the vdp.log file located under the directory <DENODO_HOME>/logs/vdp
In the Execution Trace, you may look for the errors that will help you to understand this issue.
I found the relevant info in the following user manual.
* [Execution Trace of a Statement](https://community.denodo.com/docs/html/browse/8.0/vdp/administration/creating_derived_views/querying_views/execution_trace_of_a_statement)
Hope this helps!