Not 5000 sleeper cells, but 5000 individuals in sleeper cells.
I find that number very realistic.
Even if you don't count actual al Qaida members/sleepers, there have been studies that show between 10-15% of Muslims are extremists. Assuming the low end of that figure is correct, we have around 700,000 possible extremists living in this country.
Those who doubt that al Qaida placed sleepers in the country prior to 9/11 must be crazy.. they knew full well what our response would be to that morning.
Who are you and why should it matter to me what you do or do not find realistic?