I currently work at an SME, in the tech sector. However I recently was given an offer to jump to an MNC, where I will be managing a team of 45 instead of 12. How do I prepare myself for this?

just be humble and approachable and i'm sure you'll do fine :)

It's basically 4 times more.

study how you handle the 12 now, try to plan the expansion to 45.

For time and capacity not enough, time to be creative and work out the ways, trim the unnecessary activites, consolidate some and streamline them.

Honestly, it's only when you've tried it then to be able to do it. Try :)