SELECT count(*)
FROM (
    SELECT UID
    FROM `posts`
    WHERE `status` = 1
    GROUP BY `user_id`
) sub

<?php

// Eloquent Builder instance
$SubQuery = Posts::where('status', 1)
    ->groupBy('user_id');

$count = DB::table( DB::raw("({$SubQuery->toSql()}) as sub") )
    ->mergeBindings($SubQuery->getQuery())
    ->count();

$count = DB::table( DB::raw("({$SubQuery->toSql()}) as sub") )
    // ->where(..) 這裡會出錯
    ->mergeBindings($SubQuery->getQuery())
    // ->where(..) 正確
    ->count();

<?php
DB::query()->fromSub(function ($query) {
    $query->from('posts')
        ->where('status', 1)
        ->groupBy('user_id');
}, 'sub')->count();

// select count(*) as aggregate from (select * from `abc` group by `col1`) as `a`