HI

I came here 6 december on tourist visa 1 entry. Visa was valid until ~1 march.

11 january i went abroad and come back 21 january so i got 30 days stamp.

So i have to leave country 19 february but my old visa expired ~1 march

Can i go to immigration office for extension 30 days or only 7 days?

You are here on a visa exempt entry, so cannot get a 30 day extension. The most you wil get is 7 days.

Your visa for was a "single entry" and you used that single entry. That visa is no longer valid. You're here on 30 day visa-exempt status and as Mario2008 said, the most you'll get is a 7 day extension.

If your original visa had been a "double-entry" tourist visa, then you could indeed obtain a 30 day extension. Actually, you would have been stamped for 60 days on the January 21 entry with permission to remain until March 20 (give or take a day, I'm too lazy to count them on a calendar.)