I entered usa with b1/b2 visa.CBP stamped on my passport admitted houston but din't mention duration of stay.How can i sure?

Paper I-94's are being phased out. Please check this website for retrieval of I-94: https://i94.cbp.dhs.gov/I94/request.html The maximum period that the I-94 will be valid is 6 months. Hopefully you have received the full 6 months, but there is no appeal if it is a lesser period of time. You may be able to extend your I-94 with a properly filed extension of status application with the USCIS.

You must retrieve your electronic I-94 records- for yourself and each member of the family- to be sure of the exact date until which you were admitted. Mr. Arora has provided the correct link. US Customs &Border Protection has transitioned to an electronic- only system at all US airports.

You can check the exact date for your allowed stay in the U.S. by clicking the link that attorney Arora has provided.