You can buy as many buses as you want for every depot, but you have to extend the depot for more buses. So over time you will be able to get a bigger and bigger bus fleet. Always before starting a tour on a map, you have to choose a bus from the depot of the map, which you will use to drive the tour.
When buying a new bus you can choose if you would like to buy a new or second hand bus. If you buy a new one, you can exactly choose your desired bus from all of your installed buses. Basically all buses for OMSI 2 are compatible with the Bus company simulator. For second hand buses you will get different offers which are randomly generated and are only valid for maximal 24 hours. Here you can only buy certain buses to always different prices. The mileage of used buses is of course much higher than from new buses.
The repaints and license plates of your buses can be customized individually for a small amount of virtual money. Your buses lose petrol when you drive them and can get damaged at accidents in OMSI. Also without crashes it is possible in rare cases, that a part of your bus gets defective (this happens more often with second hand buses than with new ones). So you regularly have to refuel and repair your buses if required.
It is also possible to sell buses, but you will only get back a part of the original bus price.