First of all executeQuery is a method to execute a single query of type select , therefore not only is it not necessary to indicate the end of the statement with ";" but in several drivers you will get execption ORA-00911: invalid character . The reason is that ";" is not part of the syntax but a statement separator used by CLI tools. You can read more about this here
About your question:
How can I store the number that returns the select count to me in a
variable?
You can use ResultSet to store the result of your query.
In your query you know that you will get a single value and it is an integer, so the correct thing would be to access it through ResultSet.getInt (int) passing as a parameter the first index ( 1 )
public int contar_num_personajes() throws SQLException{
int n= 0;
Connection dbConnection = get_conection();
Statement stm = dbConnection.createStatement();
// almaceno resultado de consulta en ResultSet
ResultSet rs = stm.executeQuery("SELECT count(*) FROM personajes");
// chequeo que el result set no sea vacío, moviendo el cursor a la
// primer fila. (El cursor inicia antes de la primer fila)
if(rs.next()) {
//Si hay resultados obtengo el valor.
n= rs.getInt(1);
}
// libero recursos
stm.close();
dbConnection.close();
return n;
}