Which certification do you get after the 20weeks training as a 25b ?

You don't get any certifications. You have to take those tests on your own once your knowledge and experience level are high enough

None. You'll likely need Sec + at your unit but they'll handle it.

Since you are National Guard I would highly recommend getting a certification if you want to work IT on the civilian side. I personally recommend Sec+ seeing as its the preferred of the IA level 1 certifications. It may have changed but the school house will allow you to take a certification test before/after school hours, if you have completed all Skillport learning for that topic and a measure up test that can show that you are ready for the CompTIA Security+ certification test. It is not an easy certification so you will need to study, but very useful in the civilian market.

Here is my advice as an 25B S6 Battalion NCOIC as well as a network engineer supporting the USMC.
1. Earn your Security+ - you can find classes to take thru the military. Talk to your training NCO.
2. You need and OS cert. 1st WIN10 and then I would recommend LINUX. A formal cert is best but at least do the online training courses the military offers.
3. That will keep you busy for a while. Check back in when you get it done.

You get opportunities to cert of a few things but none are mandatory. You will only get them if you want to. On the other hand you cant really do your job without them.