It depends. If all of the domains are very similar, lets say they are merchants selling to consumers (B2C) and the things that users can do on each domain is similar (add/remove items to/from carts, create an account, purchase goods, etc.), then it might be best to put the domains under the same Sift account. The benefit in putting multiple domains under the same account is that all of the domains benefit directly from the information coming in from each of the domains.
However if the domains are somewhat different: For example, if some of the domains are merchants selling to consumers (B2C) and others are marketplaces (C2C) or the actions that users can take are different on each domain, then the domains should go on separate accounts. This is best because the fraud signals may be different for each domain in this case. The domains will still benefit from fraud patterns learned from each other, as well as all of Sift's other customers, through our global model!