Several crossing points are open at the border between Romania and Ukraine: Halmeu/Dyakovo,Sighetu Marmației/Solotvino, Siret/Porubne, and Isaccea/Orlivka (ferrycrossing). It is also possible to enter Romania via the Republic of Moldova. A map of the border points and real-time waiting times can be checked on the website of the Romanian Border Police - here.
In these past few days, the border points at Sighetu Marmatiei and Siret have been the most used by Ukrainian refugees.
From https://www.romania-insider.com/ukrainians-romania-entry-rules-2022