It's not just about reviews. There are lots of other metrics that come in to the equation to guarantee reaching the local 3 pack.
5 reviews on Google won't necessarily get you on top, or will 50. It might, but it's not the only thing you should aim for here. You want to get positive, GENUINE reviews from as many different places as you can - not just Google My Business.
Other important factors are a mix of local citation sources, correct NAP details, geo tagged images, website schema, domain authority etc... I could go on, but I don't want my competitors stealing our no# 1 spot
As a rule of thumb, I NEVER use services of a company that contacts me first. ESPECIALLY when it comes to SEO/Local SEO. This is your livelihood here, if they are guaranteeing something (which by the way, they can't) personally I'd steer well clear - they could potentially damage your rankings and penalise your website and that is one big sh*thole to get out of.