If I clicked on an email link can someone hack into my iPhone?

The short answer is NO, your phone cannot be hacked by clicking on a link, no matter how dangerous it looks.

In general your iPhone cannot be hacked at all, except by someone who has physical possession of it while it is unlocked.

The real risk of clicking on links is being tricked into entering personal information on what looks like a legitimate web page. Here’s some information about that→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

No. iPhones are very difficult to hack. Unless you have provided your Apple ID and Apple ID password on such site you're fine