The following will create the directory, if not existing:
<PropertyGroup>
<Number>79</Number>
</PropertyGroup>
<MakeDir Directories="minified/$(Number)/" Condition="!Exists('minified/$(Number)/')" />
<JavaScriptCompressorTask
SourceFiles="@(jsfile_global)"
DeleteSourceFiles="false"
OutputFile="minified/$(Number)/global-min.js"
CompressionType="Standard"
LoggingType="Info"
LineBreakPosition="-1"
/>