Here is the math.

Using Blk 5's that have flown only in 2018, and that only BLk'5 will fly in 2018.

That each Blk 5 will be capable of 3 flights 1 as new and 2 as used without much refurbishment work. Mostly just inspections and tests, little or no hardware swap-out.

That the initial quarter of 2018 (2018 Q1) launches 4 new Blk'5. The build rate of 1st stages remains at 4 per quarter. The build rate of US starting in 2018 is increased each quarter until reaching a build rate of 12 per quarter by 2018 Q4.

Then by EOY 2018 there will have been 32 launches, with 67% of then in the last quarter being used. This used to total rate will become steady state such that a total of 48 launches is possible in 2019.

This is without requiring more manufacturing floor space.

By decreasing the 1st stage build rate to 3/quarter and increasing the US build rate to 16/quarter. By increasing the re-flight rate for each booster by 1 to 1 new and 3 used flights for a total of 4, enables a total number of launches per year of 64 also without the need for more floor space. The used to total flights rate will then be 75%.