Welcome to the Forum.
Branch line passenger trains could be anything from 1 to 4 or more coaches long, depending on the number of passengers expected. Railway companies ran the smallest trains they could to keep costs to a minimum. So a branch line connecting two reasonable sized towns would carry more passengers than a branch that linked only small villages and would have longer trains.
For a model railway the limit is usually the space you have available for your layout. I think three or four coach trains are probably reasonably realistic without needing too much room.
Have fun!
Regards,
John Webb
PS As you've become a member, you don't need to use the Guest posting area; for questions about the real railways which you want to ask, there is a 'Prototype' section in the index you could use. It's worth looking at as you might pick up some ideas to help you design your layout. And in the Track, Layout and Scenery section a lot of people have asked about their layout design as well.