To anyone that have an idea please help me;
I have a 19 unit of aircondition and the capacity are 5 ton each and with a 3 phase power supply, I have to use a GENERATOR SET. How many amperes of generator should I use.

Your question cannot be answered as stated.

First Amperes depend on voltage. Your AC could be 208, 460 or whatever.

Second, Power is expressed in Watts or in the case of motors kVA. VA is Volt-Amps which is to one aproximation Watts.

Third, the requirements will DRASTICLY change if you assume that all the motors can start at once. I'll let you read the first page, here: Phase Converters & Electric Motor Starting Currents - Phase Converter Information It's not directly related to generator sizing, but LRA or locked rotor amps would be required to size unless things like variable speed drives or some sort of sequential starting was employed on the motors.

So, at a minimum the starting currents of all the motors. Not a fun scenereo. Internal short cycle delays (built in variable delays (2-5 minutes) to turn on after an off cycle) can cause issues too.

Costs will be significantly different if they all have to start at once and you don't sequentially start them. You also have to take into account the air handler. Distribution may also have to include other voltages and moving backed up loads to a transfer switch/panels.