Seeking advice on whether there should be a single platform for investing in stocks/ETFs (local/overseas), and which is the best broker?

Any advice on saxo vs IB, and whether its risky to use a non-SG broker? The fees are quite complicated and i'm not sure if there will be hidden fees. (So far, i see that there are trading fees (per trade) and monthly maintenance (?) fees.

Also, what is a good amount for a single stock to diversify my portfolio? How do i start diversifying when i have yet to invest?

Any other tips for beginners? especially if per trade will likely be < 10k, total portfolio < 100k for the next few years..