How many bits are used for a subnet ID in an IPv6 address?

48 bits is the length of the global routing prefix portion of an IPv6 address, not the subnet ID.

An IPv6 address is 128 bits total and is divided into three logical parts: a 48-bit global routing prefix assigned by the ISP or registry, a 16-bit subnet ID used by organizations to segment their network, and a 64-bit interface identifier for the host. The 16-bit subnet ID field allows an organization to create up to 65,536 subnets within its assigned prefix.

64 bits is the length of the interface identifier (host) portion of an IPv6 address, not the subnet ID.

128 bits is the total length of an entire IPv6 address, not just the subnet ID field.

10 bits has no standard allocation significance in the IPv6 address structure defined by RFC 4291.