The Number property should hold the numeric value of the string parsed from the stream. When a numeric value (either an integer or float) is found in a stream, the parser creates a token and sets this property to the value parsed. It also should set the CommonInterfaces.IBaseStreamToken.DataType property to the correct numeric data type.
Setting the Number property should not set the DataType property unless there is only one numeric type for that parser. Usually, there are at least two numeric types (integer and float) so the DataType should be set separately from the Number property.