You can modify the number of filters for all layers by by reducing the depth_multiplier scalar in your config (found under the feature_extractor heading).
Alternatively, you can specify the number of filters in the individual layers which provide feature maps for the localization/classification layer by modifying the feature_map_layout in the relevant feature_extractor module.
For your example you would modify the object_detection.models.ssd_mobilenet_v1_feature_extractor.py module and change the layer_depth. The defaults are shown below.
feature_map_layout = {
'from_layer': ['Conv2d_11_pointwise', 'Conv2d_13_pointwise', '', '',
'', ''],
'layer_depth': [-1, -1, 512, 256, 256, 128],
'use_explicit_padding': self._use_explicit_padding,
'use_depthwise': self._use_depthwise,
}
A preferable alternative would be to create your own feature_extractor module, ensure it is imported in object_detection.builders.model_builder.py and modify the type in your config under the feature_extractor header.