Sometimes one does not need more than 1 cylinder.
The earliest steam engines were single cylinder (working copy of Newcomen’s 1708 engine; Cornish pumping engine; Bolton & Watt beam engine; paddle steamer engine) and large single cylinder stationary engines were in production until the 1930s (Robey uniflow steam engine; Blackstone gas engine).
Nowadays single cylinder engines are common in small motorcycles, outboard motors, generators and hand-held machinery (chainsaws etc).
Single cylinder engines have two advantages: simplicity and efficiency. Thermodynamically, any heat transfer process in which heat is conducted across a temperature difference is irreversible: it leads to an increase in entropy and is a source of inefficiency.
Any cylinder strong enough to withstand the internal pressures without significant distortion (which would lead to leakage around the piston) must be fairly thick. This means that the cylinder wall cannot go up and down in temperature as the gas temperature changes: there always tends to be a cyclical temperature difference between gas and metal. The compression and expansion processes (ideally isentropic) inevitably result in the gas changing temperature - in fact from the steady flow enegy equation we know that power output depends on the rate of enthalpy change, mdotCpDeltaT.
One can reduce the heat loss by running the engine hotter (e.g. adiabatic Diesel engines, insulating the cylinder of a steam engine) but even so, there will be heat fluxes into and out of the gas during the cycle. Heat input during compression raises the work required; heat loss during expansion reduces the energy extracted.
Increasing the cylinder size increases the ratio of volume: surface area so heat transfer has less of an effect on the thermodynamic cycle. This helps large Diesel engines (e.g. 1 m bore, 4.5 m stroke) to have efficiencies of 50-55% whereas smaller Diesel engines might have an efficiency closer to 40%.