Please define what you mean by "32 bit host" - 32-bit hardware, or 32-bit host OS?
If hardware, then NONE. If host OS, then ALL/It Depends - refer to knowledgebase article 1901 for details on host hardware requirements for running 64-bit guests.
As suggested, please read knowledgebase article 1901.
Hardware and Firmware Requirements for 64-Bit Guest Operating Systems (1901)
Basically, the host OS does not matter. But you must have a 64-bit processor with certain requirements explained in the article.