Reminds me of default passwords on wifi routers a decade ago - ATT especially had a very identifiable SSID format (ATT###), and a default 10-digit password. That leaves you with (9,999,999,999 + 1 =) 10 billion[1] passwords possible, which even at that time only took a couple hours to test all of them. That SSID pattern also left you with only 1,000 possible SSIDs, so a rainbow table was definitely reasonable.
[1] - though now that I think about it, that might not properly cover the case of leading zeroes in the password, so the total number of possible passwords might be larger than 10B; that's assuming a naïve password list generated just from numbers, not from treating the digits as characters, so I need to reason about this a bit more...
It's O(10 billion), so your intuition is good regardless :) passwords with ten 10-digits: 10x10x... = 10^10 = 10 billion, passwords with nine digits = 10^9, etc etc down to 11,111,111,110 (I don't think we should count the empty password). The full length password dominates the size of the keyspace so much that you more or less get truncations for free.
[1] - though now that I think about it, that might not properly cover the case of leading zeroes in the password, so the total number of possible passwords might be larger than 10B; that's assuming a naïve password list generated just from numbers, not from treating the digits as characters, so I need to reason about this a bit more...