I would like to know if I have subdomains to my main domain, do i have to create seperate txt records?

say i have example.net which i send email from google.

example.net TXT v=spf1 include:_spf.google.com ~all

but also have several servers that send email using their local MTA.
I have 2 servers called sms1.example.net and sms2.example.net, they are on subnet 1.1.1.0/24 and 2.2.2.0/24. Taking that into account I expanded my TXT to

example.net v=spf1 ip4:2.2.2.0/24 ip4:2.2.2.0/24 include:_spf.google.com ~all

I am not sure if the above is sufficient becuase when root user at sms1 sends email it sends from root@sms1.example.net.

using all of the spf record testers out there, they verify for example.net, but not for sms1.example.net, it says no spf record found.

Do I have to create SPF record for every single machine I have ? can I include it all in one SPF record to handle all sub domains as well?

Regards,

That looks like it should work. I would create an mx record for each of the servers and then add 'mx' to your spf record and then test the results. Also don't forget to allow time for propagation.