OSX El Capitan reset the number of Spaces
The list of spaces is stored in the following file.

/Users/YOURUSERNAME/Library/Preferences/com.apple.spaces.plist

For example on a machine with the dashboard and three desktops:

&lt;key&gt;Spaces&lt;/key&gt;
&lt;array&gt;
    &lt;dict&gt;
        &lt;key&gt;ManagedSpaceID&lt;/key&gt;
        &lt;integer&gt;3&lt;/integer&gt;
        &lt;key&gt;id64&lt;/key&gt;
        &lt;integer&gt;3&lt;/integer&gt;
        &lt;key&gt;pid&lt;/key&gt;
        &lt;integer&gt;59241&lt;/integer&gt;
        &lt;key&gt;type&lt;/key&gt;
        &lt;integer&gt;2&lt;/integer&gt;
        &lt;key&gt;uuid&lt;/key&gt;
        &lt;string&gt;dashboard&lt;/string&gt;
    &lt;/dict&gt;
    &lt;dict&gt;
        &lt;key&gt;ManagedSpaceID&lt;/key&gt;
        &lt;integer&gt;1&lt;/integer&gt;
        &lt;key&gt;id64&lt;/key&gt;
        &lt;integer&gt;1&lt;/integer&gt;
        &lt;key&gt;type&lt;/key&gt;
        &lt;integer&gt;0&lt;/integer&gt;
        &lt;key&gt;uuid&lt;/key&gt;
        &lt;string&gt;&lt;/string&gt;
        &lt;key&gt;wsid&lt;/key&gt;
        &lt;integer&gt;1&lt;/integer&gt;
    &lt;/dict&gt;
    &lt;dict&gt;
        &lt;key&gt;ManagedSpaceID&lt;/key&gt;
        &lt;integer&gt;4&lt;/integer&gt;
        &lt;key&gt;id64&lt;/key&gt;
        &lt;integer&gt;4&lt;/integer&gt;
        &lt;key&gt;type&lt;/key&gt;
        &lt;integer&gt;0&lt;/integer&gt;
        &lt;key&gt;uuid&lt;/key&gt;
        &lt;string&gt;514D761A-D08B-402E-BF8E-FFB607A340EF&lt;/string&gt;
    &lt;/dict&gt;
    &lt;dict&gt;
        &lt;key&gt;ManagedSpaceID&lt;/key&gt;
        &lt;integer&gt;5&lt;/integer&gt;
        &lt;key&gt;id64&lt;/key&gt;
        &lt;integer&gt;5&lt;/integer&gt;
        &lt;key&gt;type&lt;/key&gt;
        &lt;integer&gt;0&lt;/integer&gt;
        &lt;key&gt;uuid&lt;/key&gt;
        &lt;string&gt;CF552E87-9A59-41C7-B0AF-7AEFFB1DFB43&lt;/string&gt;
    &lt;/dict&gt;
&lt;/array&gt;


Unfortunately OS X holds a copy of the file in memory, so if you just delete it or modify it, it doesn't make any difference as it just recreates the file with the same number of spaces.

You could log into another account, delete the file from the Terminal, and then log back into the affected account.

Or you could just create a new account for yourself and use it, and leave the 325 spaces for your colleague if they return. Each user account has its own com.apple.spaces.plist file.