When people start beekeeping they usually get one hive of bees. Once they get their bees and install them into their hive, they start running into problems. The queen isn’t a strong egg layer, the bees abscond, the bees swarm, they get sick and die off, they bring back pesticide from flowers and the hive dies, the queen dies, or any number of issues. Eventually the hive is too far gone and can’t be recovered.
You can try to order another queen to replace a lost one or another package of bees. But the demand for these things is very seasonal and demand far outpaces supply. You may find that it takes too long to arrive and by the time it does, your hive is not recoverable.
I recommend having at least two hives. This way if one queen has issues you can get rid of her and install eggs from the other hive. If the queen in one hive dies, you might get lucky and the bees may start making a new queen in the original hive, but if they don’t start immediately, they will run out of eggs and lose the opportunity to make a new queen. If you have access to another hive with a good queen, you can take a frame with eggs on it and place it in the other hive.
If you plan to keep your operation small, I would still recommend building your operation up to four hives. This way you have several hives to choose eggs from if one gets weak or doesn’t have a queen. It is possible for one hive to be weak and another to have a queen die on you. But if you have four hives, it is unlikely that all of them will be having issues at the same time. Try to get to four hives as quick as you can with your splits. Once you have four hives you should have a self-sustaining operation and shouldn’t need to buy any more bees.