Hello, I am working on a project and i need to control 8-9 sg90 servos. Can I connect them directly to arduino? If i will use 2 2100mAh batteries to provide power to arduino, I have to connect the possitive of battery to Vin and Gnd to Gnd?
Do mean 2 Lipo batteries in series, so 7.4V nominal? If so then yes connect to Vin and GND.
You can CONTROL the servos from an Arduino. But you cannot POWER them from the Arduino 5V pin. They need to be connected to the battery, through a 5V DC-DC converter. For that many SG90s it should be able to supply at least 5A (the stall current of an SG90 is around 0.6A).