The Texas Department of Criminal Justice has a free public offender search tool on their website that makes this straightforward. Go to tdcj.texas.gov and look for the offender search function. You can search by name alone if you do not have any other identifying information. The results will show your husband's TDCJ number, current facility, sentence information, and projected release date.
If the name search returns multiple results, you can narrow it down using date of birth or county of conviction to make sure you are looking at the right person.
Once you have the TDCJ number, keep it somewhere accessible. You will need it for adding funds to his phone account, setting up visitation, sending mail through InmateAid, and communicating with the facility about anything related to his case. It is the one identifier that follows him regardless of where he is housed within the Texas system.
If for any reason he does not appear in the TDCJ offender search, it may mean he is still in county jail awaiting transfer to a state facility, or that the system has not yet updated his record following a recent intake. In that case calling the county jail or the TDCJ classification office directly will get you the number faster than waiting for the online database to catch up.