Static Maps API has URL length limit of around 2048 characters. You can hit this limit quickly when adding lot of markers. You can keep URL short by clustering markers together.
Square Based Clustering
Clustering is usually done by dividing map to squares. Square size depends on map zoom level. Markers inside a square are then grouped into cluster. This technique has some limitations. Look at the following image.
Two markers are close to each other. In fact they are so close they are overlapping. Both markers are also the only marker inside their square. Because markers are in separate square they wont be clustered.