U could also do this using PowerCLI:

First connect to your Horizon Environment:

Connect-HVServer <yourserver>

Then you could do something like this:

$ViewAPI = $global:DefaultHVServers[0].ExtensionData

$query_service = New-Object "Vmware.Hv.QueryServiceService"

$query = New-Object "Vmware.Hv.QueryDefinition"

$query.queryEntityType = 'SessionLocalSummaryView'

$Sessions = $query_service.QueryService_Query($ViewAPI,$query)

$NonWindowsClients = $Sessions.Results.namesdata | ? {$_.ClientType -notlike "Windows"}

$NonWindowsClients.count