Yes he can. But what you have now is a game of political chicken. One of the few gentleman's handshakes that still exist is just the justices the US has. Since 1869, it's been 9 of them. But in the past there were as few as 5 and as many as 10.
There is no law that says you can't have 17. Biden could literally appoint 8 more democratic life appointed judges if they got through the confirmation process.
But then the day Trump or the next republican is elected, they can just appoint N-1, 16, republicans... Then when it flips again, 32 more democrats... It would never end. The court would be proven to be political, even though that facade is crumbling with the ignorant American masses finally.
What happened is two of them died, one retired, opening up three spots. One actually died during Obama's term, but the Dems let the Republicans delay the appointment thinking Hillary was a shoe in and then lost that position. Stronger governance and they could've appointed one of their own before Trump. Also RBG dumbly (and selfishly) did not retire under Obama, another Dem misstep out of weakness and unnecessary righteousness.