There is no practical limit to the number of U2F-secured services the FIDO U2F Security Key and other U2F-certified YubiKeys can be associated with.
During the registration process, the key pairs are generated on the device (secure element) but the key pairs are not stored on the YubiKeys. Instead, the key pair (public key and encrypted private key) are stored by each relying party/service (such as Google) that initiated the registration. Therefore, this approach allows for an unlimited number of services to be associated with the U2F-certified YubiKeys.
To see where else you can use your U2F-certified YubiKey, see our FIDO U2F page.