Can I say “An alumni of x university recommended this course to me”, if there is only one person who recommended me the course?
You can and people do, but they’re mistaken.
The singular is alumnus for men and alumna for women. The plural is alumni for men or mixed groups and alumnae for women. It’s from Latin and common enough that alumnuses is going to get you a side eye but you run the risk of coming across as undereducated if you use the plural form for a single person.
That said, it’s a minor bit of grammar in a non-English language and it’s not going to cause any major problems unless the prof already dislikes you.