https://codex.wordpress.org/Class_Reference/WP_Query#Properties
found_posts
is the total number of posts returned by the query.
example code:
<?php
$args = array( 'post_type' => 'your_custom' );
$custom_query = new WP_Query( $args );
if( $custom_query->have_posts() ) {
$number_of_posts = $custom_query->found_posts;
while( $custom_query->have_posts() ) {
$custom_query->the_post();
if( $number_of_posts == 1 ) {
the_content();
} else {
the_excerpt();
}
}
wp_reset_postdata();
}
?>