A new bicycle chain should not automatically be cut to match the physical length of the old one. As its joints wear, the old chain becomes longer even though its link count stays the same. Park Tool explains that, if the original sizing was correct, the chains can be aligned rivet by rivet to size the replacement. When that starting point is uncertain, chain length should be established afresh for the particular drivetrain.
A chain that is too short can prevent access to the largest gears and put the rear derailleur under strain. An overlong chain can hang slack, make noise and impair shifting. There is no safe universal number of extra links to add: full suspension versus a rigid rear triangle, and the type of chain connector, both affect the calculation. The instructions for the specific component combination therefore matter more than a rule of thumb.
Before joining the chain, check once more that it runs correctly through the rear derailleur and that the connecting parts match the chain. If the original length is uncertain or the drivetrain has special requirements, a mechanic can check the setup before a mistake damages components. Once fitted, keeping the chain clean and lightly lubricated helps limit wear on both the chain and the sprockets.