It seems to me that most people can not understand what change different amount of bet numbers.So look to this drastical example - say we bet on all 37 numbers, this way with every spin we lost 1 chip - that is equal that we simply give for casino 1 chip without fight. Next - we cover 36 numbers, here we in 36 cases we end on 0 and in one case we lost 36. Again there are no fight - we cant win but rare can lost but many. Third option - we bet 35 numbers - here we will win 1 chip in 35 options from 37 , but in 2 options from 37 we will lost 35 chips.
Now lets say we bet in every case two times , then in first case we always end with -2, in second case we in 95% end with 0 , but in 5% we end with -36 or with -72. In third case we in 90% end with +2 but in 10% we end with -34 or -70. Now one more case we bet 30 numbers - here after two spins we can end in 81% with +12 and in left 19% we can end with -24 or -60. If you will look to max and minimal values you will see that with increasing positive value - negative value is decreasing, but in the same moment with increasing positive value chance to get it - decreasing and oposite for negative value - with decreasing value chance to get it - increase.If we go further in our analysis we will find that if we bet 26 numbers we have about 50% chances to end in plus +20 and in left 50% end in minus -16 or -52.
If we will put all chances and values to one we everywhere will get the same result, what means that in long way there are no diference how much numbers we play if our edge is negative, so this case for us is much profitable to bet as less as possible - to have small chance to win much in theese several spins and not risk big money.
But can be some cases that we need some amount of money which is less than our bancroll, here we can choose betting number amount to have maximal chances to achieve our aim.
This all phylosofy was about that case when we have -1/37 negative edge against casino, and in this case we must play as less as possible. When we have possitive edge - all is oposite - we must bet as many as possible, but here are also a hint. As I say all theese examples are drastical and I only give them for better understanding - not sugest to play that way.
But when say we have advantage and say we know 30 numbers in which ball will stand we can bet them all and have sure +6 profit but we can bet more say 32 this way we decrease our profit or bet less say 20 to increase profit to 16 but left 1/3 when we will get hit in our area but we lost.