The number of columns shown in a sub category page is controlled in Mosets Tree's template CSS. By default, the location of your Mosets Tree's template CSS file is at the following path:

/components/com_mtree/templates/m2/template.css

To change the number of columns, look for the following declaration block:

#subcats li {
	width: 32%;
	padding: 0;
	margin-top: 10px;
	float: left;
	background-image: none;
	}

Note the width declaration. The value 32% tells browsers to use 32% of the total width for a single

...

tag. This will produce a maximum of 3 columns before it moves to print the next

block in a new row.

If you wish to change the number of columns, change the width declaration's value. For example, changing the value to 20% will use up to 5 columns, while a value of 25% will use up to 4 columns.