If, on the other hand, you have indecisive players who spend 5 minutes on each combat turn and need time to look their powers up, don't plan on more than one encounter per session.
I'd say you should plan 3 encounters ahead. You don't need to shove them all into the session, you can just leave an encounter for next week or even drop it entirely if the players run into an unexpected direction.
A few sessions ago I was planning a fight where I expected the group to be overwhelmed and flee, but they decided to take the big bad guy head-on. It was a tough battle and the group only barely defeated him, with one PC down, and the others in single-digit hit-points. They only won because the two PCs that remained standing both rolled crits in the last turn.
This one fight took the whole session, but the players had a blast so that's perfectly okay.
A couple of tips to make fighting faster...
- Have people roll multipul d20s if they're making multipul attacks using color-coding.
- Make sure people are prepared for their next turn. I always try to have my next plan of attack in the next round ready to go when my turn comes up.
- Call out the the roll of Initiative. This is to make sure people know when they're going so they can prepare.
Just some tips I picked up and I think they're useful in almost any campaign.
As for how many encoutners for your first session. I'd have to go with 4. This will let you be prepared for almost anything and if they do something that's not in the campaign, then resort to the Monster Manual to decide on what to throw at them next.
3 encounters in a combat intense game
2 encounters in a roleplay intense game
2.5 encounters per 4 hours
you level every 9 encounters (+1 for finishing a quest)
Prepare 10 encounters ahead if you have time and you are set for an entire month of once a week games.
Using stock monsters and guidelines from the MM and DMG respectively, it will probably take about 20-25 minutes to set up each encounter if you toss in some descriptions and pre-pick loot. After some practice, you may get it down to 10-15, but then you start making your own monsters... and... well... time investment spirals upward...
For about 4 hours of work on your own, you can prepare 16 hours of gaming. Take an afternoon, leaf through some books, let your imagination run wild and see what seems like an interesting place to throw your players!
I find the best combat and RP experiences come from games where I have player characters with backgrounds in my hands before I even begin planning, then I can customize it to each of them and explore their stories as the plot itself! Fun!
Hope this is in some way useful to you.
In my very first battle as a DM with my inexperienced players, I had three brigands challenge the party. However, after about 4 turns, I had another thug appear from the rocks by the side of the road, and used a Charge to attempt knocking down a player. The Smart Thug used his Dazing attack to delay the Aegis-utilizing Swordmage, since special effects bypass the 5-point damage defense. In this way, I was able to show players the nature of battle without really injuring their characters.
Be sure to have players create powercards or print them out of the DDI builder if they have it. In fact, lend your computer to them if you can, so they can use the builder to make their character. Powercards are very useful, especially when mixed with computerized accuracy. They can help make your battles a bit faster, since people won't be flipping through books as much. The Dungeon Master's screen is also recommended, since it puts most of the conditions and such in front of you.
In short, it is important to make your battle information simple enough for players to digest, so you could do more playing and less research on the fly. Make up rules that keeps players from doing things that can disrupt your game. EG: When a rules dispute comes up, give your judgement as a DM and tell players you will look it up after the session, or disallowing leveling up characters in mid-session.