Is that true that WP_Query by default returns 10 posts? I have more than 10 posts and didn't set the post_count variable and it returned 10 posts. Then I set post_count to 1000 and it returned all posts (Now I have 27 posts for test purposes). Then I read some documentation and learned from there that I can set post_count=-1 to return all posts. I tried and it worked. But I couldn't find an explanation if it's fine that it returns 10 posts by default. Thanks.
Yes, the default is 10 posts. You should have a look at the documentation for The Loop. That will help you get a better understanding of how this works.