How to limit homepage to one post

don’t edit the template files;
try working with 'pre_get_posts' – https://codex.www.ads-software.com/Plugin_API/Action_Reference/pre_get_posts

nearly relateded example: https://codex.www.ads-software.com/Plugin_API/Action_Reference/pre_get_posts#Changing_the_number_of_posts_per_page.2C_by_post_type

ideally, start by creating a child theme;
then add the corresponding code into functions.php of the child theme;

example code:

function special_home_pagesize( $query ) {
    if ( is_admin() || ! $query->is_main_query() )
        return;

    if ( is_home() ) {
        // Display only 1 post for the original blog archive
        $query->set( 'posts_per_page', 1 );
        return;
    }
}
add_action( 'pre_get_posts', 'special_home_pagesize' );

Is it difficult to create a child theme? I went on the link you provided, but it seems so complicated for a newbie like me.

Can I just edit the existing functions.php file and it would work so long as I don’t update my theme?

And is there a specific place I need to add it to the functions.php file or can I just throw it in at the end?

edit the existing functions.php file

you can do this – just remember not to upgrade your theme, or any changes will be lost (irreversible).

yes, you can add the new code at the end of functions.php of Twenty Ten

Thank you, the code worked perfectly! ??