The Federal Energy Policy Act of 1992 required all faucet / shower fixtures made the USA to have a flow rate of no more than 2.2 GPM at 60 PSI. Often you can get below 2.2 GPM with low flow aerators but before 1992, older fixtures used much more water than 2.2 GPM. To determine your required GPM, add up how many fixtures of what type you will have served by the tankless water heater:
As you can see, older pre-1992 faucets and shower heads can require very large water flow. So to accurately size your water heater, you need to measure the actual water flow from your faucets and shower heads which is outlined in this quick tip Quickly Determining GPM Flow Rate.
I really recommend you go through that simple exercise for bathroom faucet, shower head and kitchen sink faucets, even relatively new ones. Since the Federal Energy Policy Act of 1992 is not heavily monitored, there have been problems with faucets exceeding their stated 2.2 GPM ratings. If that's the case, you need to know before you size and install your tankless water heater.