Hi,
So I think I have spotted the issue , the counter was counting any number of devices so I have added a filter to only count Windows Devices, give this ago :
Select Top 1000000 Query1.Username,
  tblAssets.AssetName,
  tblOperatingsystem.Caption As [Operating System],
  tsysAssetTypes.AssetTypename
From tblAssets
  Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
  Inner Join (Select tblAssetUserRelations.Username,
        tblAssetUserRelations.AssetID
      From tblAssetUserRelations
      Group By tblAssetUserRelations.Username,
        tblAssetUserRelations.AssetID) Query1 On Query1.AssetID =
    tblAssets.AssetID
  Inner Join tblOperatingsystem On
    tblAssets.AssetID = tblOperatingsystem.AssetID
  Inner Join (Select tblAssetUserRelations.Username As UserName_Count,
        Count(tblAssetUserRelations.AssetID) As Counter,
        tblAssets.Assettype
      From tblAssetUserRelations
        Inner Join tblAssets On
          tblAssets.AssetID = tblAssetUserRelations.AssetID
      Where tblAssets.Assettype = -1
      Group By tblAssetUserRelations.Username,
        tblAssets.Assettype
      Having Count(tblAssetUserRelations.AssetID) > 1) Counter On
    Counter.UserName_Count = Query1.Username
Where tblAssetCustom.State = 1
Group By Query1.Username,
  tblAssets.AssetName,
  tblOperatingsystem.Caption,
  tsysAssetTypes.AssetTypename,
  Counter.Counter
So this will show
UserName	AssetName	Operating System	AssetTypename	
Mr A             Asset1          Windows 10              Windows
Mr A             Asset2          Windows XP              Windows
But will not show 
UserName	AssetName	Operating System	AssetTypename	
Mr A             Asset1          Windows 10              Windows
Mr A                     Mon1                    NA                         Monitor