What is the minimum MTU of IPv4 68 bytes or 576 bytes?
That is really an invalid question that is mixing terms from two different network layers. Network (layer-3) protocols, e.g. IPv4, do not have MTUs. An MTU is the Maximum Transmission Unit that a data-link (layer-2) protocol on a specific physical (layer-1) medium can transmit (hence the Transmission in MTU) in its payload (the layer-3 packet).
IPv4 will have a minimum and maximum packet size. The minimum packet size will be the header (20 to 60 octets, with 20 being the real modern header size) plus the payload (UDP has a minimum datagram size of eight). The maximum IPv4 packet size is based on the 16-bit Length field, which means 65,536, and that is larger than any real data-link protocol MTU.
In the modern world, there are no data-link protocols with an MTU as low as 576. It could be possible to configure that on an interface, but why? It would also be possible to get that low with tunnels inside of tunnels, inside of tunnels, but that would be a bad setup that should not be tolerated. In reality, IPv6 requires a minimum MTU in its path of 1280, so that is likely the smallest MTU you will encounter in the real world.
Really, the 576 number is now historical trivia (off-topic here), but a node, such as a router, must be able to accept packets at least that large, so setting an MTU smaller than that is really not allowed.