Ventilation for garage conversion

I converted my old concrete garage into a home office - I put celotex insulation panels all over the walls, then plasterboard, and then had them skimmed. I also had an insulated steel roof put on, and new UPC door and windows. Because I'm an idiot, I didn't really think about ventilation, and now that I'm working in here all the time I realised that this little room seems pretty airtight now. Although the walls are sectional concrete panels, the roof gables are just wood cladding on the exterior, so I was thinking I could easily cut a hole through and put a vent on either side (probably 300 by 150 mm). Will just one vent be enough to let fresh air in, or do I need to install a couple so air can flow in one and out the other?