Also, a reseller purchased from the US are generally cheaper compared to SG as the cost of bandwidth is cheaper in the US than in SG.
I don't think there is a set number of clients that will determine when you should move from reseller to a VPS or even a dedicated. What determine this should be the kind of site(s) you will be hosting. If all of your clients are just small time bloggers, you may perhaps be able to host even more than 30 clients without any worries but if you had a business client like WHT then you will definitely require more than just a reseller account.
Good luck with your venture