Since he already has 9 wins and might get another 26-27 starts, it seems possible. But since 1980, only one pitcher has won 25 or more games, Bob Welch, who won 27 in 1990.
I looked at all the seasons since 1946 with 25+ wins. Those pitchers averaged 38.5 starts per season and 308 IP. Webb's career high in games started is 35 and for IP it is 236.33. Mel Parnell, in 1949, had the lowest number of starts for a 25 win pitcher (33). But Parnell won one game in relief and had 295 IP.
The pitchers who had 35 or fewer starts (Webb's career high) that won 25+ games averaged 274 IP, well above Webb's career high of 236. This group of pitchers also averaged 37 games pitched, so they had 2.5 relief appearances on average, which could help them win an extra game or two (that group of 6 pitchers averaged 34.5 starts). Webb has only pitched 1 game in relief in his entire career.
The lowest number of IP for a 25 win pitcher since 1946 was Welch's 238 in 1990, just a bit more than Webb's career high. If Webb were to make 26 more starts, he would have to win 16 of those games or 61.5% of his starts. In his career, he has won 43% of his starts and last year he won 52.9%. He has been allowing 2.98 runs per 9 IP and the Diamondbacks are scoring 5.4 runs per game. Using the Bill James Pythagorean formual, that works out to a winning percentage of .767.
If he were to get the decision in 82.3% of his starts (his % from last year) the rest of the way (26 starts) and if he had a .767 winning percentage in those games, he wins 16.4 games. Added to the 9 he already has, he gets to 25. So he needs to keep pitching as well as he has this year and the Diamondbacks need to keep scoring 5.4 runs per game.