You need to pass the max instance number as an hparam when the model is  created. Set the MAXOBJECTCOUNT slightly higher than the max number of objects you expect to see in an image.
from tflite_model_maker import object_detector
object_detector.EfficientDetLite0Spec(hparams {'max_instances_per_image': MAXOBJECTCOUNT})

All kinds of model parameters can be tweaked using the hyper params dictionary.
You can read more about them here 6.