Ludwigsburg is fantastic, but can be confusing. There is an entry fee for the grounds, which is what most people do. This allows you to walk all around the gardens, which are extensive. This includes the greenhouse, the aviary, the children's area, the Italian Garden, the castle tower (not part of the Schloss), and the exotic bird cages. All of this is very popular and I highly recommend you go in April/May when everything is blooming. Around Easter they have special decorations for the holiday, and as the warmer weather arrives they have more outdoor events. Weekends can get crowded with families. It's much more a local place than a tourist destination, which is why there's only two English tours daily.
Don't miss the 200 year old merry go round, or the flamingos in the aviary, if you have young kids. The beer garden in that corner is good, and popular.
There are two tours, which is where you see the two prices. One tour for each side of the Schloss; the Kings side or the Queens side. Each side is different, and the tours take about an hour each. If you do both, and walk the gardens, you're going to be there most of the day. It's worth it.
When you enter the grounds you have a long walk up to the Schloss. At that point you can go into the courtyard or around the building. To the right is a path with numerous food stalls which leads to the gardens on the backside of the building. The Italian Garden is to the left. Inside the courtyard is the gift shop. That's where you buy your tickers for the tours. Do this when you first get there, because if you're on the back of the building you cannot enter the courtyard without a ticket, and it's a long walk to go around just to meet up with a tour.
There's another cafe in the courtyard, and it's a good place to grab a snack, meal, or just a drink while waiting for a tour time. Just make sure you let the waiter know you have a time to meet, as service can be slow when it's busy.
I've never seen the English tours sell out, usually it's a group of less than 20. The tours are a lot of walking, and little children can have issues with that. Especially if they've done a couple miles around the gardens.