So Trump has appointed Gorsuch to replace Scalia, essentially a wash, and Kavanaugh to replace Kennedy, which should tilt the balance. (Kennedy was a GOP appointee, but never acted as one).
The real juice comes when he replaces the lefties.
RBG is hospitalized and at her age whether she leaves the hospital is an open question. She wasn't a fortress of health before her hospitalization, and her addmitted problem controlling her drinking (passed out during the State of the Union address) makes you wonder how she fell.
There is not much law, either empowering or limiting, that addresses the President's ability to nominate Justices. FDR famously threatened to pack the court by increasing its size beyond 9. (There is no law that says it is limited to 9). Similarly, there is no law that says the President can't appoint a temporary Justice to fill in for RBG while she is in the hospital or unable to stay awake.
Looking ahead, Sotomayer is 64 but a Type 1 diabetic. She joined the court 9 years ago but seems to have doubled in weight since then. I can't see her staying on the court past 2024.
Kagan is probably the other true lefty, but she is only 58. While she looks like a lesbian, and may be one, she was previously married to a man but is now divorced. She might just be an incel. Some speculation here:
https://abovethelaw.com/2010/05/its-offi...-straight/ Although she has admitted to smoking as a teenager, she seems reasonably healthy and unlikely to step down until there is a Democtrat in office.
Finally, Breyer, 80, is a moderate/liberal appointed by Clinton. That means he's anti-conservative. He seems reasonably healthy, is married to a woman and has 3 kids and probably several grandkids. He likely won't retire until there is a Democrat, although he might be 86 by then.
Note that being on the Supreme Court is like being a professor. You have summers off. You have clerks to do a lot of the work for you. So working that late in life is common.
Finally, Clarence Thomas is a trustworthy conservative vote but he is 70 and overweight. Being black, having grown up poor, and suffering significant stress in his life including an interracial marriage, you have to equate those 70 years to a much higher number. Would not be surprised to see him retire during Trump's first term to allow Trump to appoint an equally trustworthy replacement. They hire clerks for an annual term, so not likely until next summer.
So it very possible Trump replaces 2 more justices during his first term (RBG and Thomas), and a reasonable possibility of replacing Sotomayer and Breyer eventually, which would make 6, with an outside chance at Kagan for a 7th.