Is there a limit on how many we can draft of each position?
Sorry for being slow to reply. No, there is no limit to how many you draft at any given position. You need to fill a team of: QB, RB, RB, WR, WR, TE, W/R/T, K, DEF. The W/R/T slot is filled with the WR, TE, or RB of your choice. If you want to try and draft the best WRs during the draft to get somebody to trade later, you can do that. Everybody has their own strategy, but there tends to be a premium on RBs.
There are a few things out there to help you with who you draft. A lot of it is knowing who starters are, what offense they are in, who's a TD machine. I know I scored big last year with just finding out that Julius Thomas was the starting TE for the Donkeys, and picking him up as nobody knew who the hell he was. That's why I try to draft so much later, so that teams can be mostly finalize their rosters, and injuries from pre-season are dealt with.
As far as injury swapping: if one of your players gets injured during the season, you have a choice to make: keep them on your team, or drop them and try to find a replacement from the FAs available. Everybody gets the positions listed above and 5 bench slots, so generally most people have somebody to plug in when there is a late scratch(nothing like having your starter being a late scratch in a late game...).
You can pick up any player that has not played yet for the week. If they have played, they are on Waivers until Wednesday. On Wednesday, players that are claimed are doled out according to each person's position on the waiver list. You claim a player from Waivers, and you go to the bottom of the list. It can be key sometimes to save your Waiver until that one breakout player is found.
I'm sure there's more info to be had about things, and I'll be glad to chat with anybody about the nuances of the rules. I will be in the live drafts as well, so feel free to grill me for any more info you desire at that point. I can be PM'd with any questions you may have at any time, and maybe we could setup a time to just use the chat here to answer any questions people think of? Let me know.