Everyone above is giving you good advice.

Based on your culvert sizes, it looks like you may be buying them at a "big box" store. I was evaluating a similar purchase and decided to calculate Price per Square inch of culvert area. In my case, the largest culverts were clearly the best deal.

Further, the work to lay and pack one 24" culvert is much less than the work to do four 12" culverts.

Finally, hydraulic calculations are very difficult, even just to acquire the proper data. However, you can "poor boy" your calculations a little bit. If the last few floods were flowing over the road in a stretch 10' wide and 6" deep, then that is another 720 sq. inches of flow you need to bypass. (Plus some extra cushion since flow through pipes is a little more restrictive than open flow.)

Two 24" culverts (452 sq. in. each) would cover that estimate.

The final problem is that the largest water flow you have witnessed, is almost certainly NOT the highest possible water event for your crossing - so you still need a little extra cushion on top of your calculations.

P.S. There is a chance that the larger culverts will not "fit" under your current road bed. In that case, look at the elliptical or "pipe arch" shapes for your application. The pipe arch shape really lets through a lot of water while minimizing your height restrictions.