It's a OOTB vApp limitation. Please do the following to adjust the limitation settings:
1. On IM Management Console, export the UserStore directory xml
2. Adjust maxrows setting on the DirectorySearch section to a larger number, for example: maxrows="50000"
<DirectorySearch minsortrules="1" maxpagesize="1024" maxrows="10000"/>
3. save the xml
4. re-import the User Store directory xml
5. ssh to vApp machine as config user
6. su - dsa
7. vi /opt/CA/Directory/dxserver/config/limits/vapp-default.dxc
8. locate the following line:
set max-op-size = 10000;
9. change the setting to a larger number, for example:
set max-op-size = 50000;
10. save the change and exit vi
11. dxserver init all
After these, the bulk task preview screen should be able to show correct number of the population.