A YubiKey is a hardware security key that can be used to protect access to online services, computers, and networks. It's designed to simplify the two-factor authentication (2FA) process, which is an extra step in the login process that verifies your identity. YubiKeys are embedded with a unique code that generates authentication codes when you press a button on the device.This allows you to securely log into your accounts with a single press, making the login process more efficient and secure.

Verify yubikey: https://www.yubico.com/genuine/

https://github.com/drduh/YubiKey-Guide

• https://developers.yubico.com/Software_Projects/
  • https://developers.yubico.com/yubikey-manager-qt/
  • https://developers.yubico.com/yubikey-manager/ [Guide]
• https://github.com/OpenSC/OpenSC

YubiKey 5 Series firmware 5.7+

• FIDO2 - the YubiKey 5 can hold up to 100 discoverable credentials (AKA hardware-bound passkeys) in its FIDO2 application.
• FIDO U2F - similar to Yubico OTP, the FIDO U2F application can be registered with an unlimited number of services.
• PIV - the YubiKey 5's PIV (smart card) application has 4 usable slots per the PIV specification, each serving a specific purpose (authentication, digital signature, key management, and card authentication - click here for further information). In a Microsoft Windows environment and used in conjunction with the YubiKey Smart Card Minidriver, the number of usable certificates expands to approximately 12 (dependent upon many factors including algorithm used as well as various Certification Authority settings).
• OATH-TOTP - the YubiKey 5's OATH application can hold up to 64 OATH-TOTP credentials (AKA authenticator codes).
• OTP - this application can hold two credentials. However, Yubico OTP, one of the most popular kinds of credentials to put in this app, can be registered with an unlimited number of services.
  • The OTP application comes with:
    • Yubico OTP
    • Challenge-Response
    • Static Password
    • OATH-HOTP
• OpenPGP - the YubiKey 5's OpenPGP application can hold up to 3 subkeys (signature, encryption, authentication) linked to a single OpenPGP identity.

YubiKeys 5 Series with firmware 5.0-5.6

• FIDO2 - the YubiKey 5 can hold up to 25 discoverable credentials (AKA hardware-bound passkeys) in its FIDO2 application.
• FIDO U2F - similar to Yubico OTP, the FIDO U2F application can be registered with an unlimited number of services.
• PIV - the YubiKey 5's PIV (smart card) application has 4 usable slots per the PIV specification, each serving a specific purpose (authentication, digital signature, key management, and card authentication - click here for further information). In a Microsoft Windows environment and used in conjunction with the YubiKey Smart Card Minidriver, the number of usable certificates expands to approximately 12 (dependent upon many factors including algorithm used as well as various Certification Authority settings).
• OATH-TOTP - the YubiKey 5's OATH application can hold up to 32 OATH-TOTP credentials (AKA authenticator codes).
• OTP - this application can hold two credentials, can be registered with an unlimited number of services.
  • The OTP application comes with:
    • Yubico OTP
    • Challenge-Response
    • Static Password
    • OATH-HOTP
• OpenPGP - the YubiKey 5's OpenPGP application can hold up to 3 subkeys (signature, encryption, authentication) linked to a single OpenPGP identity.

https://support.yubico.com/hc/en-us/articles/360013790319-How-many-accounts-can-I-register-my-YubiKey-with