if you do 21tech you will need 8 rax to have same ammount of knights as 18 tech 4 rax at 8:20. 8:20 50 food knights coming at you nigger. 16 food have first knights at 7:00, 18tech at 7:40, 21 at 8:20. 3 Different knight waves. 40seconds apart each. Technicaly 16 food could have 48food knights (8knights) at 7:40 if you had 4 rax, but you just dont have enough resources for that shit and its just wayyyy to unsafe. 18 on other hand, gives you early rax, you can have about 2 and mb 3 towers and you have spare wood for an upgrade or some militia use. Those early raxes serve as defence, if you get all inned you can make footmen easy.
and bonus old bloodmage special creeping technique on EI