Based on the novel 20 Times A Lady by Karyn Bosnak, What’s Your Number? centres on a 20-something career girl who imposes a strict limit on the number of men she is allowed to sleep with before she finds her Prince Charming.

When she hits that limit – 20 lovers – the only choice for the beleaguered heroine is to revisit her old flames, hoping that she missed a diamond in the rough the first time around.

Gabrielle Allan and Jennifer Crittenden’s script clings to this flimsy dramatic conceit like a first-time swimmer with a flotation device.

Alas, Mark Mylod’s flat-footed and hare-brained romantic comedy capsizes within the first 10 minutes.

Anna Faris is a gifted comic actress but she can’t perform miracles and sadly Mylod’s flaccid film gives her nothing to work with.

Ally Darling (Anna Faris) reads a newspaper article that suggests the average person has 10.5 partners in their lifetime and single women are doomed to spinsterhood if they reach 20 sexual partners without hearing a peal of wedding bells.

Shocked to discover that a recent drunken fumble with her boss takes the notches on her bedpost to 20, Ally enlists the help of bed-hopping next-door neighbour Colin (Chris Evans) to re-evaluate the suitability of her old flames as partners.

Fast-rising politician Tom Piper (Anthony Mackie) is a perfect candidate except for one little stumbling block: his sexuality.

“America’s ready for a black President. It’s not ready for a gay black President,” he confides, suggesting Ally could date him to enhance his public profile.

Predictably, the man of Ally’s dreams might be someone closer to home.

The film starts poorly and the tumbleweed keeps rolling across the cinema as the leads gamely attempt to milk laughs, even in sympathy, from the clumsy script. Some of the humour misses the mark entirely.

When one old flame tells Ally “I may not have been your first but maybe I’ll be your last”, she tastelessly responds “Why, are you going to rape and kill me?”

Evans loses his shirt, which seems to be his chief talent nowadays, while Faris humiliates herself during a disastrous encounter with British beau Simon (Martin Freeman) by adopting a faux English accent that slowly veers into Eastern Europe before settling close to the manic Swedish chef from The Muppets.

On-screen sexual chemistry between Faris and Evans is inert, leaving us to wonder if Ally might be destined for the emotional scrapheap after all.