Best amount of combat encounters for a 6 player lvl 3 party

Soon, I’ll be hosting a D&D session for six 3rd-level characters, but I was having some difficulty on deciding the amount of encounters.

Everyone has some experience with tabletop gaming, but there are three players completely new to D&D. I want to show off the system with some interesting and challenging encounters.

I’ve been using Kobold Fight Club to calculate the difficulty, but have heard it is calculated with 6-8 combat encounters a day in mind. As things are, I have five encounters planned which all seem to be hard to very hard . After certain points the players could decide to take a long rest for resets. This makes it especially hard to decide on a difficulty

One battle, for example, is a fight with a hydra that will most likely get the drop on them. It is ranked “very hard”, but with six players I’m quite sure they will defeat the hydra relatively quickly, even with its regeneration.

To avoid making the game longer, and to leave time for roleplay, social encounters, and shopping I’d rather refrain from having too much combat during each session.

How would you scale challenge rating and the amount of encounters in one adventuring day with a party like this? Or will the difference not be as great as I imagined?