Real Madrid goals can reach 3-4 goals against Betis. Real Betis is not even up to Real Madrid standards to start with this season, they're completely below elite clubs this season and that simply means they don't pull any significant threats against elite clubs. Carlo Ancelotti have worked out his plans and he's just aiming for the possible best results in matches to be played by the club he has worked so hard to manage. This is supposed to be a game we shouldn't be talking about because I'm pretty sure of Real Madrid will record massive winnings and three points is fully secured.
Why are you so confident that Real Madrid can score more than 3 goals? 
Whatever your reason, I don't think Real Betis is a mediocre or weak team. In fact, they are still in 7th place although you said they are below elite clubs in La Liga. On the other hand, Real Madrid is also not in their best shape, there are some players who will be absent in the next match. Vinicius, Camavinga, and Tchouaméni won't be among the players in the match against Real Betis. It may decrease a bit the strength of Real Madrid.
I don't know how many goals to be made on the match. I think it is something unpredictable. But since Real Betis isn't a weak team, I don't think there will be a massive wins or many goals.