modified version of https://gist.github.com/juavidn/80a8b5cc755330120b690a82469fbfe2

LimitPerGroupable.php

<?php

namespace App\Traits;

use DB;
use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Query\JoinClause;
use phpDocumentor\Reflection\Types\Mixed_;

trait LimitPerGroupable
{
    /**
     * @param Builder $query
     * @param int $limit
     * @return mixed
     */
    public function scopeLimitPerGroup(Builder $query, $limit = 15)
    {
        $judgement = !$query->getQuery()->wheres ?: $query->getQuery()->wheres[0];

        // check if query is actual eager load
        if (!$judgement ||
            (!strpos($judgement['column'], '.') && !isset($judgement['values']))
        ) {
            $query->limit((int)$limit);
            $query->getModel()->setPerPage((int)$limit);
            return $query;
        }

        /** @var JoinClause $join */
        $join = $query->getQuery()->joins[0];

        $table = $this->getTable();
        $eagerLoadFullColumn = $judgement['column'];
        $bindings = $query->getBindings();

        // make sure column aliases are unique
        $numAlias = "{$table}_rn_" . uniqid();
        $groupAlias = "{$table}_grp_" . uniqid();

        /** @var Builder $newBaseQuery */
        $newBaseQuery = $this->newQueryWithoutScopes()->getQuery();

        $subQuery = clone $query;
        $subQuery->select("{$table}.*");

        // initialize MySQL variables inline
        $subQuery->from(DB::raw("(select @num:=0, @group:=0) as vars, {$table}"));

        // apply mysql variables
        $subQuery->selectRaw("@num := if(@group = {$eagerLoadFullColumn}, @num + 1, 1) AS {$numAlias}");
        $subQuery->selectRaw("@group := {$eagerLoadFullColumn} AS {$groupAlias}");

        // make sure first order clause is the group order
        $subQuery->getQuery()->orders = (array)$subQuery->getQuery()->orders;
        array_unshift($subQuery->getQuery()->orders, [
            'column' => $eagerLoadFullColumn,
            'direction' => 'asc'
        ]);

        // for many to many table relationship
        if ($join) {
            $subQuery->addSelect($eagerLoadFullColumn);
            $newBaseQuery->joins = $subQuery->getQuery()->joins;
            $eagerLoadColumn = explode('.', $eagerLoadFullColumn)[1];
            $inspectColumn = "{$table}.{$eagerLoadColumn}";
            $newBaseQuery->whereRaw("{$inspectColumn} = {$eagerLoadFullColumn}");
        }

        $query->setQuery($newBaseQuery);
        $query->from(DB::raw("({$subQuery->toSql()}) as {$table}"));
        $query->setBindings($bindings);
        $query->where($numAlias, '<=', (int)$limit);
    }

}

Could you explain how to use this scope?