Can I fit 28mm tires on this bike? It’s a Jamis Ventura Comp : https://www.bikepedia.com/QuickBike/BikeSpecs.aspx?item=23765
Thanks!
Maybe.
Given your link, it came with 23mm tyres.  You may still have that size, read the side wall to confirm.  If your current tyres are a different size, then this example will be off.
A 23mm tyre is about 23mm wide, and a 28 is.... 28.  There is some variance on size because of rim width.  So ideally you would use a vernier caliper to measure the maximal width of your tyre, when on the rim and inflated to the highest pressure you would typically use.  Lets assume they measure out at the nominal size.
Your planned 28mm tyre is 5mm larger than a 23mm, so it will use 2.5mm more in width on each side and about 5mm more in height.  The height is on one side because the rim doesn't move, whereas the width change is on both sides of the center line.
With your wheels and 23mm tyres in the bike, find something that is 2.5mm thick.  A hex driver/allen key is a good candidate, or a sliver of wood like a chopstick, or some plastic of that thickness is good too.
Feed this feeler-gauge around your bike frame.  Specific pinch points to check are

Chainstays, both sides
Seat stays, both sides
Fork tines/legs, both sides
Mudguard/fender stays, both sides

Now double over your feeler gauge so its 5mm thick, and check

Rear brake caliper, underside
Brake bridge over the rear wheel, underside
Front brake caliper, underside
Fork crown, underside


If your feeler fits everywhere, then a 28mm tyre will &quot;fit&quot;
HOWEVER a close tolerance is not necessarily good - you do need additional space for dirt/mud, and simple flex while riding.  I would recommend an additional 2mm as a bare minimum.
I have a 90s Cannondale which really should have a max of 25mm tyres.  I have a 28mm in the rear, but refitting it after a puncture means a 3-handed dance between the QR, the rear brake lever, and pushing on the rim at the chainstay to get it all centered.   Otherwise the bike will subtly rub and erode my wheel and left chainstay over time.
Also, if you have rim brakes then a wider tyre can be difficult to fit when inflated.  That same bike has problems getting the tyre through a 105 rear brake caliper even with the brake's QR opened.  It's easier to inflate the tyre with the wheel re-installed.