One must remember, the Constitution was written at a time when it was considered both impractical and unnecessary for Congress to be in session perpetually. The relevant rules (no citations) are:
1. Congress has to at least meet once a year, on the first Monday in December.
2. Neither house can adjourn for more than three days without the consent of the other.
3. If the two houses disagree on a date for adjournment, the President can adjourn Congress until whatever future date he sees proper (no President has ever done this).
4. If Congress is adjourned, the President can call them back into session (last time this was done was in 1948).
SCOTUS has ruled that only an adjournment that is long enough to require the consent of the other house qualifies as a recess. Because of the convenience of modern travel, Congress essentially has the option of staying in session perpetually, by making sure enough members of each house are in DC in order to hold pro forma sessions every three days. This was the method the GOP used to block all recess appointments near the end of the Obama presidency.
However, so long as both houses are of the same party as the President, it's unlikely they will be too scared of recess appointments, and if the Senate does go into recess, the President can fill vacancies as he sees fit. Note, however, that any such appointment is temporary, and lasts until the end of the next Senate session (which essentially means anyone appointed this year would need to go before the Senate for approval before the end of the 2018 session in order to keep his job past that).