1. Before the fact, one should not recite the blessing if he does not know which number of the count it is, because it is forbidden to pause between the blessing and performing the mitzvah longer than the time-period oftoch k’dei dibbur (which is the time it takes to say the words,“Shalom alecha Mori veRabi”). But, after the fact, if the person recited the blessing with the assumption that he will overhear the count from his fellow, and he did in fact hear and count accurately, then he has fulfilled the mitzvah (Shulchan Aruch 489:5 and Mishnah Berurah 489:5:29).
2. If, while reciting the blessing, the person was under the impression that it is the fourth day of the Omer, and after the blessing he realized that it is day five, and he counted “today is five days…,” he has fulfilled the mitzvah (ibid.).
