This is just my guess for how they've designed Mania and Forces...
Sonic Mania:
Seven bosses
Six zones with two acts each, then one final zone with one act and Robotnik as the final boss.
That means Sonic Mania has thirteen acts in total.
Sonic Forces:
Now the villains have been revealed, I'm going to say there's six bosses (Chaos, Shadow, Metal Sonic, Zavok, Robotnik, Inifinite) each with their own zone with three acts - one act for each Modern Sonic, Classic Sonic and custom. That's eighteen acts in total with a big fight featuring all three in the games final boss.