Hi Do i have to use separate elastic search for datahub or can i use the existing one already in use

You can use an existing elasticsearch cluster as long as it is version 7.X

i'm planning on using aws elastic search (open search). Can you please share the load i can expect on the elastic search from datahub

That depends on the size of your data ecosystem. How many entities (datasets, tasks, etc.) are you planning on ingesting into datahub?