There are good official resources that state that recommended power supply must be 3 amps (e.g. official RPi power supply via mains). However, for use 'on the move' or remote locations this may not be practical. 5v 3 amp USB power banks exist, but are expensive (compared to 'standard' cousins). It seems that RPi doesn't need 3 amps for booting up or use minimal attached devices that are not power hungry (WiFi might still be okay), although I haven't tried this. I have a number of power banks around, one that outputs around 2.1 amps max. (12,500 mAh), 5v USB. Assuming no power hungry devices are required, could I boot up RPi and use GPIO (e.g. weather sensors) or USB webcam? There must be in no way any resulting damage to GPIO pins/modules/devices, SD card or RPi itself from insufficient power/amps; on-screen power warnings okay though. I have a multimeter to test devices if necessary. I assume it would NOT be recommended to attempt to boost amp output (i.e. to 3) using at least two power banks and correct circuitry to achieve this.
There are many questions here regarding powering the RPi from a battery. On the basis of your question, I will guess that you have not read many of them. For example, you don't seem to know how much current the RPi will draw in an "idle/doing-nothing* state. We welcome all contributors here at RPi, but we prize the ones who invest some time and effort to understand the subject matter. You should read some of these other questions, to familiarize yourself with the problems of powering the RPi from a battery.
In general, powering the RPi from a battery is not an easily viable proposition. Why? Because what you need is energy - power delivered over a period of time long enough to accomplish "something useful". The difference may be understood by reading a good explanation, or with some relatively simple math:
Power = V × I ; where V is dc volts, and I is dc current
Energy = P × t ; where P is power, and t is time
These two simple equations allow you to calculate how much energy you need for your "on the move or remote" Raspberry Pi application.
Figuring batteries is a bit more more complex. Batteries generate power from a chemical reaction. There are limits on the amount of power this chemical reaction can produce, and increasing this limit with any given "chemistry" inevitably involves increases in battery volume, weight and cost. There are also "wear-out" mechanisms involved in all (current) batteries that limits their useful life. However, one need not master all of these details to understand the limits for powering a Raspberry Pi from a battery!
Most batteries have various ratings. One of these ratings is amp-hour capacity. If we assume the battery has a constant voltage, Amp-hours may be used as a substitute for an energy rating. Consider this battery for example:
Battery Capacity (mAh) 26000 (or, 26 Amp-hours)
The rating implies that this battery has the capacity to produce 1 Ampere of current for 26 hours. Unfortunately, it's not quite that simple. You may wish to read this article at Battery University to gain more insight into this battery rating. But to keep things simple, we will apply an estimated de-ratiing at 75% of the manufacturer's claim:
Adjusted battery capacity = 26 Ah × 0.75 = 19.5 Ah
At 6 volts, this yields:
Energy = V × I × t = 6 × 19.5 = 117 watt-hours
You can now take this figure, and use it to estimate how much battery you need for your application. Of course you will need to know the Raspberry Pi's energy requirements to compare against a particular battery. From all of the above, it should be clear that requirement may be estimated from the equation for Energy; e.g. 5V × 1A × 24 hours = 120 watt-hours.
Hopefully, this gives you some insight into the subject, and an approach to use for answering your own question. But please do not assume this is all that needs to be done to power an RPi from batteries... we have not even touched on the subjects of regulating battery voltage to feed to the RPi, how to execute a graceful shutdown before the battery is depleted or battery re-charging techniques. Once again: The RPI was designed to run from a mains-powered USB/5V power supply; powering it from a battery is not an easily viable proposition. It can be done of course, but you will need to work through some details.