If you have a young tree and you want to grow the trunk, should you be pruning?
Generally speaking, how much should you prune in a year if growing a tree on?

Approaches vary. More foliage = more trunk. Large cut take time to heal though.

I cut back once the plant is 2 or 3 metres tall, depending also on what the trunk look like and whether the roots need trimming.

Harry
They best way to answer your question is in this link.
www.bonsaiempire.com/blog/bonsai-trunk-creation
The quickest way is cut and grow mentioned in this article.

Is there a specific tree that you are trying to thicken?

Hi Bonsai Bender,
Well it is a five year old Acer palmatum. I have been growing it in a 2 litre pot and it was doing well.
I was told the internodes were too far away from each other. But I wanted to make sure the trunk grew as thick as possible and worry about branches later.
I'm a little stuck on what technique to choose?

Harry
You seem to be very educated in your way of thinking. The methods in the article I sent you are primarily about working on Taper/\ and not so much thickening the trunk [||]. The pot it is in will restrict the growth as well. The best thing to do (if you can) is plant it in the ground or raised bed. If this is not an option then as it grows transition for a 2 liter pot to 3 liter, then to 4, 5, and so on. You do not want to jump from 2 liter pot to say a 5 liter. The more leaves you have provides the JM with more energy to grow. The more you fertilize it the more vigorous it will grow (getting a thicker trunk) but the internodes will be further away from each other. There is a method to alleviate this when you are ready to work on branch ramification. Here is a link that explains it well. www.bonsai4me.com/SpeciesGuide/AcerPalma...edGuide%20Page2.html
In regards to taper JM's are difficult. They really want to be power polls ||. Cut and Grow is my method of choice but as leatherback mentioned the wound will take time to heal over. Bend and Grow would be my second choice (if I was a little more patient). Sacrifice Branch method will not work for JM.
Let me know if this raises any other questions.

I am very excited because I might be moving really soon. The place I have will have a garden, I have been a little stuck on fertilizer. It's just that everyone has an opinion on what fertilisers to choose. I know everyone has their opinion but I will just make sure I go for the cut and grow method.
what fertiliser do you choose for solely growing a trunk as large as possible? or is that a silly question?

I do not explicitly fertilize. Trees in the ground. During the year when cleaning out the composter they get compost. When i add generic garden fertilizer to the veggies, the trees gat a handfull too.

Ok fantastic, I'll be aware when growing in the ground. I am looking to start composting, it is a really good idea.
Thanks for all the support and help Leatherback and Bonsai Bender. I used some bone meal to start off with and had that amongst some fo the roots. I am going to use Blood, fish and Bone and hopefully get a little bit more growth than last year. With that, I am using seaweed fertiliser to spray the foliage too.