How many led can connect in series with ac supply?

Usually you'll need 100ohm resistor for each led. In this kind of setting, you will get the LED intensity nonuniform because of its differences due tolerance value. the best thing is to use LED driver IC. Another benefit of using LED driver is, you can mix red led with other color.

Now, if we wanted to connect a red LED with a forward voltage of 1.8V and a current of 20mA directly to the transformer output then use the following equation:

R=V/I = 9 / 0.02 = 450 Ohms.

You could use 470 ohms from the E12 resistor series.