You can put any number of li-ion cells in series – you could put thousands in series if you wished to, and create a (very dangerous) multi-thousand volt battery. BTW, a single 18650 is, as your details correctly include, called a “cell.” The definition of a “battery” is a number of cells in combination.
That said, you are asking the wrong question. Proper design of a system combines cells to make a battery that provides the appropriate voltage and current for the load you intend to power.
E-bike motors and controllers will have ratings from 36 to 60 volts. 36 volts and 360 watts are a common variant. The 360 watt rating implies a current of 10 amps.
Li-ion 18650 cells are about 3.6 volts under load, so you would need 10 in series. But you cannot stop there – you also need to consider the current drain. A single series string of 18650 cells could probably supply 10 amps for a short while, but that would discharge the battery very quickly, and possibly reduce their life. In 2017, as I write this, 18650 cells for sustained long use should be discharged at not much more than 3–5 amps. That means for my E-bike example, you would need 2 or 3 parallel strings of 10 cells, for a total of 20 or 30 cells.
Tesla cars have motors designed for far higher voltages and currents. The battery in their P100 has 16 modules of 516 cells for a total of 8,256 cells. I don’t know the exact arrangement, but Tesla puts ~ 100 cells in series and creates a battery voltage over 400 volts when fully charged.