Timerone library help please

I have an Arduino UNO and pretty much lost on navigation. I found a sketch in a book on using the Timer1.initialize() and Timer1.pwm(). All good and fine, I can get the frequency and pulse width. My question: Can I run more than one frequency at the same time? I'm so new at this I don't know where to find info/documentation on this. I see there are other timers like Timer0 and Timer2, Timer3. Do I need to import the other libraries or can i just write a line like Timer2.initialize() and Timer2.pwm() to run more than one frequency at the same time? Thanks.

The TimerOne library isn't a standard library of the IDE but can be installed by the integrated library manager. But you have to install it and explicitly include it, it's not available by default. Timer0 is used for time keeping purposes by the Arduino core.

Can I run more than one frequency at the same time?

By "frequency" you mean "PWM frequency"? Yes, you can use up to three different frequencies but that may lead to other difficulties.

Please tell us what you're trying to achieve. If these questions are just academically, please consult the datasheet of the ATmega328p.