The reason why not, is that you have no stored water tank, so the maximum amount of water that can can come out of all the outlets (taps, showers, WC cisterns, washing-machine filler) at any time is the amount of water that comes into your house through the supply pipe from the water main.
You can measure it by running the kitchen cold tap into a bucket and timing how long it takes to fill.
For example, if you have a typical half-inch or 15mm pipe, you might get somewhere in the region of 10 lites/2 gallons a minute. This is quite reasonable for a shower. However if you tried to take two showers at the same time, they might get half each, which is rather poor, or one might get most of it and the other a weedy trickle. The same will happen if someone turns on a hot or cold tap, or flushes a WC, or the washing machine starts to fill, while you are showering.
If you are lucky enough to have an ususually large supply pipe, which continues in large pipes inside your house, then you will get a greater flow. One way to do this is to dig a trench between your house and te pavement and lay a new pipe which you pay the water company to connect to the main. You may also need a larger meter if the old one is small and constricts the supply.
There are also ways of storing water and using a pressure accumulator, which will improve flow until it is used up. This is modern advanced plumbing and not very cheap.
IMO a combi boiler is very suitable for a small home, preferably with a single occupant, but becomes less suitable the more people there are who want to use water at the same time.