Cities: Skylines - Remastered

This solution worked for me, thanks.

So there's confusion out there about what actually happens for the trucks to count. I've tested this out now and can confirm the following. Your service point will ALWAYS be 0/X on its own. The service point itself does NOT have its own trucks. The trucks that will make your counter go up are from trucks that come from a landfill or incinerator plant, no others will count like the waste complex from the Sunset Harbor DLC or recycling plants. Turn off everything that isn't a landfill or incinerator. One incinerator can put out 27 trucks on its own, but you will need at least 2 since the mandatory service point can take 3 trucks and deduct it from the dedicated garbage points and they don't count for the achievement. I got this achievement on the X1 version, and what I noticed from there is that you want the trucks to be on the road for as long as possible so the counter will stay up to at least 25 trucks, so you will need to place the incinerators as far away from the service points as you can.

Also another thing that I can't really test, but the examine text for the service point states that "collects garbage from buildings on pedestrian streets", and this could indicate that you need most, if not all, your buildings on a pedestrian street for the points to accept their trash, which would accumulate more, which would mean more trucks.