count, countif

count

IntuneDevices
| where TimeGenerated > ago (30d)
        and isnotempty(OS)
| summarize arg_max(TimeGenerated, *) by SerialNumber
| summarize count() by OS, SkuFamily, JoinType

count

IntuneDevices
| where TimeGenerated > ago (30d)
        and isnotempty(OS)
| summarize arg_max(TimeGenerated, *) by SerialNumber
| summarize Count = count()

countif

IntuneDevices
| where TimeGenerated > ago (30d)
        and isnotempty(OS)
| summarize arg_max(TimeGenerated, *) by SerialNumber
| summarize Count = countif(OS != 'Windows')