Staying with Schuman et al (2015) linked in this thread, they start with 2.5 x 10^5 cells and end up with 5 x 10^4 to 2 x 10^5 three to four days later. Why are there fewer cells even without accounting for any division? Because the treatment is toxic. This is reported in many papers.

I don't know what the proliferation rate is like for the cells in the conditions of that study, but apparently up to 7 divisions in 4 days is considered plausible for T-cells: http://www.ncbi.nlm.nih.gov/pubmed/17367338