In the schools donated by the project of the resources provided by ABC School Supply, how many schools are public magnet schools?
ABC School Supply is vendor_name;  public magnet school refers to school_magnet = 't';
SELECT COUNT(T2.schoolid) FROM resources AS T1 INNER JOIN projects AS T2 ON T1.projectid = T2.projectid WHERE T2.school_magnet = 't' AND T1.vendor_name = 'ABC School Supply'