Use Get-Content to read your list of computers, then iterate through each one to check the count, and if it is not correct append it to your txt file.
Something like this should do what you are asking:
$computers = Get-content C:\listofcomputers.txt
Foreach ($computer in $computers){
If ((gci \\$computer\c$\800patchfolder -recurse).Count -ne 345) {
$computer | out-file c:\incorrectsumoffiles.txt -append
}
}