last element in structure

As Adam said structures are unordered and there is no concept of last.
But it looks like you are trying to output a list of elements by looping
over the structure and do not want a comma after the last element. Your
if logic is the problem here, it will always be true.

<cfoutput>
<cfloop collection="#stuff#" item="i">
'#stuff #'<cfif StructCount(stuff) lt 6>,</cfif>
</cfloop>
</cfoutput>

StructCount(stuff) is always five. It the size of the structure is not
going to change while you are looping over it. You need a counter and
then compare that counter to the size of the structure to know you have
reached the end of it.

<cfoutput>
<cfset counter = 0>
<cfloop collection="#stuff#" item="i">
<cfset counter = counter + 1>
'#stuff#'<cfif counter lt StructCount(stuff)>,</cfif>
</cfloop>
</cfoutput>

This could be simplified with a list function.

<cfset stuffList = ''>
<cfloop collection="#stuff#" item="i">
<cfset stuffList = listAppend(stuffList,stuff )>
</cfloop>
<cfoutput>#stuffList#</cfoutput>

Structs are intrinsically unordered, so there's - also intrinsically - no
"last" element.

To implement ordering in a struct, you will need to do something like
include an additional "index" key which stores ordering information. This
could be a list or array of keys, in the ordering that you want
(alphabetical, chronological, etc).

structKeyList() and structKeyArray() *coincidentally* return a struct's
keys alphabetically, so one could use one of those to generate an index.
But, like I said, it's just a coincidence that they return the keys sorted.

If your data is inately ordered, you probably should be using an array, not
a struct (that's the whole difference between the two, really).

--
Adam