I read this text and dont know why in here use sind for singular subject i know 300g bis 600g is plural but when we use verb at first we need to look at subject and here subject is singular not plural:
Die empfohlene Menge sind 300g bis 600g pro Woche.
Since "sein" can take two nouns in the nominative case, there is a certain amount of ambiguity as to which of them is to be taken as the grammatical subject. In English the subject always appears before the verb, so the choice is clear, but there is no such rule in German. Instead, there seems to be a system of rules where nouns of certain types take priority over others. One of these, as noted in the comments, is that plural has priority over singular, so if there are both singular and plural nouns involved then the plural is taken to be the grammatical subject, regardless of it's position. Note that in your example, the grammatical subject is "300g bis 600g", even though it appears after the verb. There are other cases not involving plurals, and I don't know if there is a complete list of these rules anywhere. Another example is "Das bin ich." Note that the grammatical subject is "ich" even though it appears after the verb. Keep in mind that German reserves the first position for the topic, which doesn't have a grammatical meaning but indicates what the sentence is "about". That's not necessarily the grammatical subject.
