You can do it either way. All it once or one at a time. Each has its pros and cons.
Working the chakras one at a time can help you to fully explore the power of each one. The con is it is very time consuming.
Working on the chakras all at once is less time consuming, but it doesn’t allow for a full exploration of each.
There are many ways to work with the chakras and my advice would be to find what works best for you and do it regularly.
You might be interested in an older thread about how someone spent considerable time on their root and what the result was: