if i got a 32 bit MCU with a 20bit address line, how wide is the data bus?

It could be 8 bit, 16 bit or 32 bit.

eg. an MC68008 is a 32 bit CPU with an 8 bit data bus (and 22 external address lines).

The MC68000 has the same core with 16 bit data bus and 24 address lines.

The MC68020 has full 32 bit data and address busses.

The answer is in your question.

An 8 bit CPU means that there are 8 data pins on the chip and so the chip can read/write to memory 8 bits at a time. The same thing goes with a 32 bit CPU or microcontroller. So, a 32 bit CPU should have 32 data lines. There is a caveat though. Some CPUs may operate internally at a certain bit size, but reads/writes to memory are half of that. For example, the Intel 8088 operates internally at 16 bit, but it only has 8 data pins. This means that the CPU must make TWO reads or writes to memory in order to read/write a 16 bit number. The 8088 was a modified 16 bit 8086 CPU which was used in the original IBM PC and it was chosen because it made the circuit design cheaper, plus there were more parts around which supported 8 bits. The external bus slots (for peripheral cards) on the original PC was 8 bits, but the bus slots were expanded to 16 bits on the IBM AT 286 computer.

The number of address pins on a CPU/microcontroller generally indicate the maximum amount of memory that can be directly accessed. Let's say we have a CPU with 16 address pins on the chip. As you know, each pin has 2 states (1 or 0) and so 2^16 = 65536, which means that the CPU can access up to 64KB of memory. Notice that I previously used the word, "generally." I have only seen this once, but there is a possibility that a CPU can have a smaller number of address pins, even though the CPU has the ability to access more memory. This was done on the Motorola 68000 CPU. That CPU could access up to 4GB of memory, but the number of address pins on the chip was limited to 24 (16MB). Back when the CPU was designed, it was unheard of to have a computer system that had anywhere near 16MB, let alone 4GB.