In most English-speaking countries, Lee-uh or Lee-ah is the most common pronunciation.
However, in most European countries, you will most likely hear Lay-ah or Lei-ah. Here, it is typically spelled Léa or some varient to accommodate the needs of the language.
Additionally, the Hebrew pronunciation of the name Leah is Lay-ah or Lei-ah.
It is a two-syllable name - not one - despite some people's interpretation. If it is used as a one-syllable name, which would be a very rare occasion, it would be pronounced like the name "Leigh" which is simply Lay or Lei.