Database: world_development_indicators
Question: Among the countries with note on the series code SM.POP.TOTL, how many of them are in the low-income group?
SQL: SELECT COUNT(T1.Countrycode) FROM Country AS T1 INNER JOIN CountryNotes AS T2 ON T1.CountryCode = T2.Countrycode WHERE T2.Seriescode = 'SM.POP.TOTL' AND T1.IncomeGroup = 'Low income'