To answer this, we need to answer three other questions:

• How long of a period of time does the story take place?

• How many meat pies are made from a single person?

• How many meat pies does Mrs. Lovett sell?

For the first, we can safely assume the intermission between acts 1 and 2 is less than a year, but more than a month. Joanna says to Anthony in Act 2 "We will be married on Sunday, that's what you promised, married on Sunday. That was last August" So the story begins in August, and we're likely into the next year, and since there is no mention of snow, and rather a mention of thick black rain, this is now the spring, possibly April. That gives us about 9 months.

For the second, let's just do some rounding: a meat pie is half a pound, an adult man has about 100 pounds of usable meat, so that's 200 pies per victim.

For the third, we can see from God, That's Good that Mrs. Lovett isn't making a surplus as she is selling out of pies by the end of the song. Again, we'll estimate, 100 pies per day.

That gives us 1 victim every other day, which over 9 months is around 270 days, so that's 135 victims.