How many versions of VHDL are there?

There are four. The original release of the VHDL language occurred in 1987 with the adoption of the Language Reference Manual as an IEEE standard. In 1993, the IEEE-1076 standard was modified and ratified and became known as VHDL'93. This is now widely supported. In 2000, the VHDL 1076 2000 Edition appeared - this fixed shared variables by introducing the idea of protected types. Finally, VHDL 1076-2002 appeared. This includes protected mode types, but also changes ports of mode buffer to make them more usable, along with some other small changes. In practice, VHDL 1076-1993 is the current flavor of VHDL which is widely supported by tool vendors.