Trump just lowered expectations, knowing that the average seat loss in mid-terms is 60-70 seats. If the blue wave is a little ripple and the dems barely win the House, that is a victory by beating expectations. That said, the dems need 23 seats to win the House, plus the blue seats that the GOP flips.
From the attached link, the dems are ahead only 13 seats, with 36 "tossups". Assuming the tossups go 18-18, the dems still pickup 13 flips, so the total number of seats the dems can be expected to pick up is 13+13=26 seats needing 23 to win the House.
RealClearPolitics - 2018 Election Maps - Battle for the House 2018
A 4 seat majority is "expected" just from the law of averages. So we'll see which party does better or worse. Point being that if the GOP wins just 4 more seats (9 flips for the dems instead of 13) then the GOP keep control of the House, with the dems getting 13+9=22 seat gain.