In python you can limit your string by using the VARIABLE[:NUMBER] format.

First character of a string

my_string[:1]

First five characters of a string

my_string[:5]

Last seven characters of a string

my_string[:-7]

So what you need is highlighted in bold

my_name = "Sai sathi bawan"
my_initals = ''.join([s[:1] for s in my_name.split(' ')])
print(my_initals)

Output: Ssb

To force Upper Case...

my_name = "Sai sathi bawan"
my_initals = ''.join([s[:1].upper() for s in my_name.split(' ')])
print(my_initals)

Output: SSB