As for first round running backs, in recent years they haven't seen the ball as much as you would think. Here are the # of touches for first round RB selections since 2002 .
Let's dig a little deeper into those numbers.
Cedric Benson 68
Steven Jackson 153
Chris Perry 5
Willis McGahee 0
Larry Johnson 21
T.J. Duckett 139
DeAngelo Williams 154
Felix Jones 32
Rashard Mendenhall 0
Darren McFadden 142
10 of them (a little less than 50%) got ~150 or fewer carries, but each of these were what I would say are extenuating circumstances. E.g., LJ, SJax, DeAngelo had guys locked in as RB1 and were not really factors as rookies. Other guys like Felix Jones, McFadden, McGahee, Mendenhall would have been more involved I think had it not been for injury.
Jonathan Stewart 191
Chris Johnson 294
Adrian Peterson 257
Marshawn Lynch 298
Reggie Bush Saints 243
Laurence Maroney 197
Joseph Addai 266
Ronnie Brown 239
Cadillac Williams 310
Kevin Jones 269
William Green 259
So, more than half had close to 200 or more carries, and the two who were below 200 carries were both part of a RBBC and had a lot of competition for carries. The rest averaged 270 carries. Looking at his competition, I would put Wells more squarely in the second group than the first. Last year, Arizona was dead last in rushing attempts at only 306. If he got 270 carries of this, it would be 88% - probably way too high. But even 65% of the carries would put him at 200 carries. (And they had 365 carries in 2007, Whisenhunt's first year there). On top of this, there's been a lot of talk about wanting a more balanced offense, and drafting Wells is certainly an indication that improving the rushing game is important to them.
Also, consider that they were much more balanced in the playoffs. During the regular season, they averaged under 20 carries per game. In the Wildcard round, they ran the ball 37 times to 32 pass attempts. The following week against Carolina, they ran the ball 43 times to 32 pass attempts. Against Philly, 29 rushes to 28 passes. They abandoned the run in the Super Bowl, but then again this was against the Steelers defense and obviously they were forced to air it out a little more against them. Even still, they averaged over 30 rushing attempts per game in the playoffs, a full 10 more attempts than in the regular season.
And, with Whisenhunt/Grimm's roots and Wells' ability to really grind out yardage, his upside could be even higher. If they can carry over their good defensive play from the playoffs, I think the offense will be even more balanced.
Then, consider that the teams in their division are average at best against the run (with the Rams being a bottom 4 team).
Assuming he stays healthy, I don't think 250-300 carries is at all unreasonable, and especially if he does get the goal line carries then he can certainly have a fantasy impact at that level.
ETA: And just for the record, Mendenhall had 19 carries before he got hurt, not 0.