Can I use the same SIM card from my iPhone 6 for an iPhone 11?

Both use nano-SIM, so yes. The SIM cards are compatible.

Whether the SIM swap will work? That depends on whether one or both of the phones are carrier-locked, and possibly on some other details.

iPhone 11 also provides an eSIM.

Most cellular providers can transfer a telephone number to a new purchase without a SIM swap. The Apple contents-data-transfer provides this path; contents and number are swapped, without a physical SIM swap.

And some cellular vendors do charge for moving a telephone number to a new phone.

Tech Specs:

https://support.apple.com/kb/SP705

iPhone 11 - Technical Specifications - Apple

sure unless the iphone11 is sim locked to another carrier

Learn which size SIM card your iPhone or iPad uses - Apple Support