If you are using NiFi-1.7 then you can dynamically select DBCP connection pools.
Refer to NiFi-5229 Jira addressing this improvement.
If you are following this way you need to configure/enable connection pool before using them and then using one ExecuteSQL processor we are going to select the connection pools dynamically based on the attribute.
If you are using previous versions of NiFi-1.7 then we need to specify schema name for each query
We need to use those many process groups
Keep an attribute to identify which execute sql processor it should go then use RouteOnAttribute processor to route the flowfiles to the respected execute sql processor.