How many Mitzvos of the 613 Mitzvos do we keep today in exile?
Today in exile it is possible for us to keep 369 Mitzvos out of the 613 total numbers of commands. Within this number there are 99 commands that are only obligatory in circumstances that one may never enter into throughout his life, and hence he may in fact never come to fulfill them. An example of such a command is the Mitzvah not to divorce one’s wife if he was Motzi Shem Ra, or not to delay the payment of a worker. It is possible that throughout one’s life he will never hire a worker or will never be Motzi Shem Ra on his wife. The total amount of Mitzvos applicable today which are a complete obligation upon all [men] is 270 Mitzvos, which includes 48 positive commands and 222 negative commands. This total number of 270 is hinted to in the verse “Ani Yeshana Velibi Er”, as Er is spelled out Reish Ayin which is 270.
 Hakdama of author to Sefer Hachinuch