For abilities that trigger only if you control a Desert or there is a Desert card in your graveyard, one condition must be true as the ability triggers and one must be true as the ability resolves. They don’t have to be the same condition, though. For example, you could sacrifice your only Desert after the ability triggers but before it has resolved.

If an ability checks whether you control a Desert or there is a Desert card in your graveyard, having more than one doesn’t matter. Controlling one is the same as controlling five. There is also no extra bonus for both controlling one and having one in your graveyard.