I am using this function to get the amount of posts:
$postAmount = wp_count_posts( 'post' )->publish;
You've got the right idea, but just made a small typo. What you want is
$wp_query->current_post + 1
depending on wether or not you want to count starting at zero.
current_post in the WP_Query Codex https://codex.wordpress.org/Class_Reference/WP_Query