Write all events with level: debug, info, warn, error and fatal to one file:

<?xml version="1.0" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

    <targets>
        <target name="file" xsi:type="File"
            layout="${longdate} ${logger} ${message} ${exception}" 
            fileName="${basedir}/logs/logfile.txt" />
    </targets>

    <rules>
        <logger name="*" minlevel="Debug" writeTo="file" />
    </rules>
</nlog>

Create one log file for each day. The log files will have the following names (dependent of culture)

2016-06-05.log
2016-06-06.log
2016-06-07.log
...

<?xml version="1.0" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

    <targets>
        <target name="file" xsi:type="File"
            layout="${longdate} ${logger} ${message} ${exception}" 
            fileName="${basedir}/${shortdate}.log" />
    </targets>

    <rules>
        <logger name="*" minlevel="Debug" writeTo="file" />
    </rules>
</nlog>