The proper name for this plant is Schefflera aboricola
, as you know it is commonly called Umbrella tree. We have a member here who goes by that name "Aboricola" and is much more familiar with this species than I. I'm sure he will see this eventually and hopefully he can offer more detailed advice.
I'm sorry, what do you mean? How much is cutting them back hard?
As Opa alluded to this is a rather subjective term. Often this tree will get very leggy and it is not uncommon to cut the stems back to within a few inches of the soil. That would be an extreme example and your tree does not need that drastic of treatment.
The first thing you are going to want to do is to study the existing branches and determine which, if any, of those do not fit into your design scheme, they can be removed entirely. This decision is entirely up to you depending on your vision for the tree. I suggest you use both names (the proper name and the common name) and search Google images for examples of this species and other bonsai as well for inspiration. Speaking of inspiration look [url=https://www.helpfulgardener.com/forum/viewtopic.php?t=3343]here.[/url] Here is one I found:
After you have removed any unwanted branches the remaining ones can be cut back as well. This will bring the foliage in closer to the trunk and hopefully increase ramification, which is the repeated division of shoots. It probably would be better to wait until early summer to make any major changes as I presume it will respond more vigorously in natural sunlight. This gives you some time to consider your options.