Is there a way to show how many unique visitors visited my website without collecting (I didn't say reading but collecting) their IP or any data of their?
A privacy 100% safe way for who is just interested in their website's amount of visits and nothing else, so that can be GDPR complaint even if enabled by default.
Some javascript code or particular websites offering this type of service?
You could throw the visitors' IP addresses into a HyperLogLog. That will tell you how many unique visitors you have without the need to store any of these addresses.
You should probably self-host that if the goal is to protect your visitor's privacy.