I edited my post, but in case you don't see it right away:

The network depends on coins being broken down into smaller pieces, but if your coins are in too many pieces then you'll never be able to stake all your coins in a single day. Check coin control (enable it in settings > options > display tab) in the send tab to see how your coins are broken down in the wallet, and their current age.

As for how many coins to send, depends on how many you have. Whatever you do send will immediately lose all their weight right now, so you'll stop staking as much, so that will be an important consideration. I'd suggest half, but check coin control and see try and find a reasonable balance from there.