Many. If you mean how many different organisms cause pneumonia, a very large number ranging from bacteria, to viruses and fungi. A strain typically is a genetic variant of a kind of organism, the flu virus can cause pneumonia, but influenza a can have slight variations making one type of strain of influenza slightly different from another ( why every year you need a new flu shot).
8 major types, 100's. In only about 1/2 cases of pneumonia no organism is isolated. Although there are hunderds of strains of pneumonia organims, most common are: streptococcus pneumoniae(50%), haemophilus influenzae (20%), chlamydophila pneumoniae (13%), mycoplasma pneumoniae(3%). Staphylococcus aureus, moraxella catarrhalis, legionella pneumophila and gram-negative bacilli.