Custom get_queryset with LIMIT for django admin panel

I would to have a custom query to show a List of element in my admin django panel.

So, to have this I use a code like this:

class MyAdmin(admin.ModelAdmin):
....
   def get_queryset(self, request):
        return Post.objects.filter(author_type=AuthorType.USER)

This work well but I need also to add a LIMIT for this queryset:

class MyAdmin(admin.ModelAdmin):
....
   def get_queryset(self, request):
        return Post.objects.filter(author_type=AuthorType.USER)[:500]

But when I add the limit clause [:500] I have this error:

Exception Value: Cannot reorder a query once a slice has been taken.

any suggestions?