The number of features in the table. If all the features are stored in the local table, for example with a GeodatabaseFeatureTable or a ShapefileFeatureTable, then the FeatureTable.numberOfFeatures will return the full number of features.
If the table is created from a service, for example, ServiceFeatureTable or WfsFeatureTable, the FeatureTable.numberOfFeatures will only return the number of features in the local table cache. You need to populate the local table cache to return a value for FeatureTable.numberOfFeatures. Do this by rendering the features in an ArcGISMap or ArcGISScene with a FeatureLayer, or by querying the feature table. Consider the feature table's FeatureRequestMode in both cases. See ServiceFeatureTable or WfsFeatureTable for more information. If you want to return the full number of features in the service, execute FeatureTable.queryFeatureCount(QueryParameters) and pass "1=1" to the QueryParameters.whereClause.