How many Li-ion batteries can be put in a series?

You can put any number of cells in series. You could put thousands in series if you wished to, and create a (very dangerous) multi-thousand volt battery.

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 for long-term use. 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