LCLeonard Bob uses 4 different types 95% of the time:
- 1 inch brush
- 2 inch brush
- Fan brush
- Liner brush
- and his majesty - pallet knife
All brushes require very thorough cleaning (it does not mean beating! and scrapping them about the metal mesh!) to extend their lives as they are very expensive. You can read about cleaning on the forum here.
Those brushes are groomed in some special way that allow Bob Ross technique.
Liner brush is the weakest brush overall (no matter what brand it is) as it hates cleaning in the thinner only and requires a wash in the oil to remove the paint accumulated near ferrule. Oil can be even cheap cooking oil or baby oil. Also water and soap have high efficiency to remove the paint.
So in terms of quantities - get 2 of one-inch brushes, 1 two-inch brush, 2- fan brushes and liner brush.
Only liner brush does not really require to be BR brand and you can find cheaper analogue brush, just remember to have it with long bristles. Liner brush I have is synthetic and with proper care and cleaning it last for 6 months (20--25 paintings) approximately. I tried another cheap brand and it was finished after 5 paintings.
Do not buy more at beginner stage unless you feel technique (30-40 paintings). Then you can start searching for the brushes as you will be understanding more of their behaviour.
They are not required for some time, you will find only little quantity of episodes with them.
Need to mention about canvas size for all those brushes: Minimum is 16 by 20 inch (40x50 cm). The smaller will not be suitable for those brushes.
Natural brushes allow rough texture and brush strokes, synthetics allow quite smooth application of paint. All brushes will wear with time and bristles become damaged beyond any repair. More thoughtful you are with care about your brushes, less often you need to replace, but replacement is inevitable.
In general minimum is to keep two brushes of the same type one for dark colors and another for light colors, this will reduce amount of cleaning required during painting and will keep reasonable amount of cleaning after painting. Don't forget you can wipe on the towel and go in for the next color with that brush.
In short what you have in a starter kit shall be sufficient for your first 5-10 paintings.