If the choice is between an "x" or nothing, then you can use a fairly simple 3-D formula, like in B3 shown here. Make sure that there are no sheets between the start and end sheet that you don't want included.
If there could be something else in D2, then you could use a formula like in C5 here. Put the list of sheets you want to include in B5:B6 (or whatever).
Excel 2012
<tbody>
[TD="align: center"]2[/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: center"]3[/TD]
[TD="align: right"]2[/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: center"]4[/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: center"]5[/TD]
[TD="align: right"]2[/TD]
[TD="align: right"][/TD]
[TD="align: center"]6[/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
</tbody>
Sheet6
[TABLE="width: 85%"]
<tbody>[TR]
[TD]
Worksheet Formulas[TABLE="width: 100%"]
<tbody>[TR="bgcolor: #DAE7F5"]
[TH]Cell[/TH]
[TH="align: left"]Formula[/TH]
[/TR]
[TR]
[TH="bgcolor: #DAE7F5"]B3[/TH]
[TD="align: left"]=COUNTA(
Sheet6:Sheet7!D2)[/TD]
[/TR]
[TR]
[TH="bgcolor: #DAE7F5"]C5[/TH]
[TD="align: left"]=SUMPRODUCT(
COUNTIF(INDIRECT("'"&B5:B6&"'!D2"),"x"))[/TD]
[/TR]
</tbody>[/TABLE]
[/TD]
[/TR]
</tbody>[/TABLE]