There’s a difference between raunchy and tawdry, and it’s a line that this unsavory rom. com crosses all too often. It wants to be sweet and cheeky but comes off sour and tasteless instead.

Irritatingly chirpy Anna Faris plays Ally Darling, a young Boston professional dumped by her latest live-in boyfriend (Zachary Quinto) and fired from her job in advertising by a finger-sniffing boss (Joel McHale) in the first reel. But she seems much more bothered by the fact that a test in one of those ubiquitous women’s magazines indicates that she’s way beyond the average number of “lays” a typical woman has had before linking up with Mr. Right (she’s at 19 as opposed to the average of 10.5). So in a drunken moment of decision at her sister’s pre-wedding reception, she vows to curb her slutty promiscuity and not to get to twenty sexual encounters before finding her soul mate.

Her solution? Find all her previous nineteen “friends” and “re-audition” the ones who weren’t total rejects. To assist her she recruits her randy cross-the-hallway neighbor Colin (Chris Evans), a musician by trade but womanizer by choice, who as a policeman’s son claims to be expert at finding people. In return she’ll let him use her apartment as a hiding place in the mornings when he wants to disappear while his latest conquest dresses and departs.

It’s obvious that these two will eventually bring out the best in one another and prove to be the perfect match. But along the way to this foreordained conclusion we have to deal with multiple flashbacks to the younger Ally’s various encounters, lots of scenes featuring the wedding preparations of her sister (Ari Graynor) and the prodding of her divorced, pushy mother (Blythe Danner) to find the right kind of man for herself, and a few interludes with her more pleasant father (Ed Begley, Jr.). There are also a few cringe-worthy sequences showing her re-connecting with some of those nineteen “possibles,” including two featuring Chris Pratt and Anthony Mackie and a third, the longest, coupling her with a rich, handsome fellow (Dave Annable) her mother considers the perfect catch but she decides is not the one.

But most of the picture, flabbily directed by Mark Mylod, consists of the Ally-Colin relationship, and it’s peppered with sequences that are carefully manipulated to show one or both of them strip down to only the essentials—the dumbest involving a ridiculous game of “strip horse” the two play at the deserted TD Garden one night. Both Faris and the post-“Captain America” Evans certainly boast attractive physiques, but after a while the movie comes to resemble nothing more than an extended striptease.

The two leads get to show off their other talents too, of course. As far as Faris is concerned, that involves “I Love Lucy”-style slapstick, including one scene in which her hair extension abruptly catches on fire and another when she falls on her face, picks herself up, dusts herself off and speeds away to find Colin. She’s game enough, but the shtick comes off pretty weak. Evans gets to show off his ability to sing and strum guitar, but the result doesn’t suggest that a musical career is in the offing. Graynor is treated merely as a limp foil to Faris, but Danner makes the girls’ mother genuinely nasty—something that adds to the picture’s overall unpleasant tone. As to the other “suitors,” Annable is a stiff bore, and neither Pratt nor Mackie is handed much of anything to do. McHale is stuck in a particularly humiliating part, though Quinto fares little better (and is made to look rather like Adam Goldberg’s younger brother).

Visually the picture is okay, but nothing special. The basketball sequence may be dumb, but the setting is nice, and presumably the Celtics franchise got a substantial sum for allowing filming at the location. A major wag of the finger to music supervisor Julia Michels, whose choices for the incredibly large number of musical montages stuffed into the narrative by Mylod come across as especially lame.

Even in a genre notorious for its low quality, “What’s Your Number?” earns a zero.