Potentially 4. In practice, these reducing agents are often used 1:1.
The common metal hydride reducing agents, lithium aluminum hydride, ##LiAlH_4##, and sodium borohydride, ##NaBH_4##, are often used 1:1. That is, for a ketone, ##RC(=O)R’##, 1 equiv of reducing agent is added (this means that there may 3 equiv hydride left unreacted.
When you are doing large scale reductions (i.e. when you are trying to reduce ##1/2## a mol of something or more), often you have problems when you add such copious amounts of hydride reagent, because you have to work the reaction mixture up with water, and generate a shed load of hydrogen.
As a rule of thumb, when doing large reductions, it is often practical to assume that the hydride transfer reagent will deliver at least 3 equiv hydride. This makes the workup a lot more controllable.
