I want to simulate some big vessel aerated by a gas ring sparger. the sparger is a pipe with many small holes which is bended to a ring)

the gas inlet (samll holes) is very small, how can i mesh them?

If you are using ICEM, best solution will be to create seperate bodies, for the region where you want fine mesh (around inlet) & rest of your domain. Define different mesh sizes for these bodies & surfaces attached to them. But, before this have a careful look at your geometry & see if you can simplify it. I mean, have common inlet instead of many small holes. This may not be always possible. So, just have a look.

Best luck, Chirag