In a standard classroom setting, all students can begin testing at the same time on computer-based tests. However, there are myriad factors, including your school's internet speed, device, bandwidth capacity, and more, that should be considered when testing large groups of students at the same time in one testing location. Students should not experience any difficulty downloading and starting the computer-based tests. However, since there are so many technical and hardware factors involved with computer-based testing, the best practice is to stagger the start time of students selecting the test session and and downloading the test when there are more than 25-30 students testing in the same testing location.
Staggering student-testing start times means waiting to select the "Test Session" to download in large groups of 25-30 students testing in the same location by 2-3 minute intervals.
You can find more information and a diagram about staggering student-start times in the following article: https://cbtsupport.nysed.gov/hc/en-us/articles/115001543766