Not that much really. To start, abide by the "Rule of Fours": As long as you have at least four of something, it will seem like "many", as the total space and number of objects is relatively low.
Remember to have even more of your main industry cars as you will want to have both empties and full loads going to and from the industry.
Plan on having at least five locomotives running the main trains:
One scheduled train that will be going around the mainline from point A through point B to point C. This train will stop and drop off cars at designated yards and pick up the new cars from them.
One extra train parked waiting to use the main line. This train will stop and drop off cars at the designated industries and pick up the new cars from them.
One hidden train waiting to come out of the hidden yard and run on the mainline and disappear again.
One passenger train that is normally hidden and comes out runs to the passenger station and leaves back to the hidden location until the next schedule passenger stop.
A switcher or two depending on how many yards you have. The switcher will receive the main train at the yard and break it up for delivery using the extra
You'll need at least four types of freight cars, but except for one type, you'll need only one or two examples of each. The class of cars that goes to the dominant industry should have at least four
examples, so it's obvious they are "many".
Four locomotives, two passenger coaches, and ten to twelve freight cars of four types make a small layout almost at capacity. Count the number of cars you could park on all spurs, and never exceed
60% of that number on the layout at the same time! If you think it's too little variety, have them exchanged via the interchange.