You're probably mixing two different reserves.
A bank has to have a certain capital reserve itself. Its shareholders must invest $10 to allow the bank to take $100 in deposits, which it could then loan to other customers. If if some of the loans would not be repaid, the shareholders would be the first to feel the squeeze.
A second reserve is the cash reserve. The bank must indeed keep some liquidity at hand so it can pay out that $2 withdrawal from your example.
These are not the same reserves, nor do they add up. They overlap. The first reserve is typically a long-term reserve, because many loans are long-term loans. The second reserve is typically a short-term reserve, because it covers the day-to-day fluctuations in deposits and withdrawals.
In reality, calculating both types of reserves is a bit more complicated because the real world isn't entirely black&white. For the first type of reserve, the bank needs to take into account the risk of the loan. A mortgage is better than an unsecured personal loan, and a car loan is somewhere in the middle. For the second type of reserve, cash at hand is best, government bonds are almost as good as cash, and publicly traded corporate bonds are also pretty liquid (easy to sell for cash).