Changing the amount of columns
This really is up to you and your preferences regarding the design and look of your website, but in case you need to go from two columns to only one, then you must take the following steps, in case your are using the Theme Twenty Seventeen.
The two-columns layout
The below screenshot show the default configuration of Theme Twenty Seventeen, it comes with two columns as you can see.
When the two-column layout is assigned, the page title is in one column and content is in the other.
The one-column layout
As I told you before, it is absolutely up to you, whatever you prefer, use the image below to get an idea of how your website will look like using only one column.