An app_config.xml file in this project's folder.

For reference Cosmology FAQ has an example. It just needs the app name updated for any other app
http://www.cosmologyathome.org/faq.php

<app_config>
    <app>
        <name>camb_boinc2docker</name>
        <max_concurrent>1</max_concurrent>
    </app>
    <app_version>
        <app_name>camb_boinc2docker</app_name>
        <plan_class>vbox64_mt</plan_class>
        <avg_ncpus>2</avg_ncpus>
    </app_version>
</app_config>

This is what would be needed for limit # of concurrent tasks

<app_config>
    <app>
        <name>camb_boinc2docker</name>
        <max_concurrent>1</max_concurrent>
    </app>
</app_config>

To get the name, Read the app_config files from w/in BOINC Client. Check the Event log and an error will tell you exactly the name to enter in the app_config file.