You might need to run this as administrator if your task size exceeds the soft limit.
Add-PSSnapin Microsoft.SharePoint.PowerShell
$sharepointSite = "http://my.sharepoint.website";
$spWeb = Get-SPWeb $sharepointSite;
$spSite = Get-SPSite $sharepointSite;
$result = New-Object PSObject;
foreach($item in $spWeb.Lists["Tasks"].Items) {
    $status = $item["Status"].ToString();
    if($result.PSObject.Properties[$status]) {
        $result.$($status) += 1;
    }
    else {
        $result | add-member Noteproperty $status 1;
    }
}
$result | ConvertTo-Json
it should produce something like the following:
{
    "Completed":  130,
    "Not Started":  7221,
    "In Progress":  1
}