depends on how much risk are you taking i suppose?
i sent 50items 1 SKU, for my new account last time and it worked fine. and my product isnt a hot seller. so they slowly were sold. so its fine for me. only when half of it were sold I added more SKU.
How many items should I send to FBA after creating new stealth account?
i sent them 300 items like a fool and they sold 98 then decided the account should actually be a business account and suspended me leaving me waiting for 1 month before i got my stock back complete nightmare and major disruption.
My advice from personal experience now would be 50-100 items, but all up to you really. i personally only send 50 items as i couldnt afford to lose a month again.