Four

WaPo: The GOP is in a very tough fight to keep the Senate. These ad reservations prove it. By Aaron Blake

Potentially conceding one or even two states, of course, would not be ideal for Republicans. They can afford to lose four seats — or five if they can pick off a Democratic seat in Nevada. But once Republicans lose a net of four seats, they would lose control of the Senate — assuming Hillary Clinton wins the presidential election at the same time. (For what it's worth, it's unlikely the GOP would lose four Senate seats if Trump were winning the presidency, so four is really the magic number here.)