sha-256 more than 20 char
hi. im new . and i want to know
how can i set random characters . for Example :  [numeric and lower Case]
for cracking sha-256 hashes .

i want that hashcat try bruteforce for random .
for example :

6wyx0guu5zmw7hnn4zcd => hash(sha256)
2ps2605i2ffewpiqxyhq => hash(sha256)
8ns9attqvy0lm0sa2qki => hash(sha256)
tqddsr0vblapvmhh70xb => hash(sha256)

and can i use for more characters? more than length 20?
You will not be able to use hashcat directly for attempting such a large brute force keyspace. You can however use a dictionary with rules, or pipe a mask to hashcat and apply rules. Anything over 12 characters usually is not compatible to be bruteforced with a character set.

Check out the Wiki for more information on attack modes and details regarding what hash mode to use.
Attacking a password of more than 20 digits doesn't mean anything.