system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why did Voldemort not make a horcrux out of Gryffindor's sword?

He *thought* he had the real sword, protected inside the Lestrange's vault. There was no way for him to know it was fake. He always wanted to have a horcrux made out of objects belonging to each founder and he had his chance. I am very surprised when Bellatrix panicked thinking they got the real one Harry immediately thought that there must be another horcrux there instead of the sword actually being one.

We know he also thought 7 was the most powerfully magical number but he ended up having 8 anyways:

1. diary
2. ring
3. locket
4. diadem
5. cup
6. Nagini
7. him (as Dumbledore explained)
8. Harry (Which he didn't know of course but is still important)

So we know there doesn't seem to be a physical limit, and not like he hadn't gone past it anyways even by making the second one.

So why did he not complete the set once he had the chance? I am sure he cared more about such an amazing magical item than a superstitious number.
gpt
The Sword of Gryffindor has this nasty habit of being able to be pulled out of the Sorting Hat at times. Not exactly a trait you want in an item holding your soul. Voldemort had all the other horcurxes well hidden or well protected.