Answer: Based on your description it sounds like the plant may be stretching for more sunlight, or it may be one of the forms of holly that grows naturally in a tall and thin shape. If the latter is the case, regular pruning might help it become denser.
You can simply cut back all the branches by about a third--you shouldn't remove any more than one third the foliage at any one time. Each time you trim a branch tip, it will form several small shoots, creating a denser look. One caution is that if you want it to grow as a tree, you would be better off removing one of the trunks at the base and then not trimming any more. If you trim repeatedly at the base it will be thick there, but when it outgrows the reach of your pruners it will thin out again.
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