I am interested in, how many projects by length 3-4 minutes I may create and save in GB? Or, what is a size of memory here in gigabites?

At 24-bit resolution audiofiles will take up 7.5 MegaBytes per minute per [mono] track. Just multiply 7.5MB by number of tracks by number of minutes by number of Projects. Keep in mind that you should always leave around 15% to 20% free space on your internal hard disk. Also keep in mind that Software Instrument tracks don't record audio so they only add a small amount (a few MB) to the size of a Project.

You write about 24-bit resolution audiofiles. How I know, what resolution I have, or do the all Macbook Airs have 24 bit ones?

Go to GarageBand Preferences [ GarageBand menu > Preferences > Advanced > Audio Recording Resolution:] and check 24-Bit, as shown.

Yes, now I know how to choose 24 bit, but what is the capacity of memory of gb, how many gigabytes? I have used sd card for my thoughts and pictures and I know, how many stuff I can put there. Or do you want to say that gb songs will be saved to my harddisc, I mean, they will be saved to the memory of Mac?

By default GB Projects [including any audiofiles] are saved to your hard disk in ~ / Music / GarageBand. [~ is your Home folder] You can check the capacity of your hard disk from Apple menu > About This Mac > Storage.

Well, it's default. But can I change default for a new default, that all my projects will be saved in Icloud? And if they will be saved to Icloud, the projects will be not copied automatically to harddisc?

I understand, one thing is an audiofile (it means real guitar, drum?) which takes 7, 5mb per minute, but another one is software instrument file (track, song), which adds small amount mbs to the size of a project. Correct? So, if I will not use the real instruments, I don't have to worry about the use of a memory, OK? Is track and an instrument the same, and a song and a project the same?

But if I still record my voice with instruments, the song will be as audiofile?

Why is with 24-bit better than without it?