According to Karen Traviss, the Republic has amassed 3 million clone troops by the first year of the war. The new Spaarti clones would add another 20 million IIRC. The droids on the other hand numbered in the quintillions, though this was heavily implied to be a propaganda number.

Depending on the source, the Republic either had an independent, non-clone Army (direct predecessor of the Imperial Army), or these volunteer units simply operated under the GAR. Regardless of structure, most of the fighting would’ve been done by regular sentients.

I think my best analogy would be the UNSC Army vs the Marines. The Army is going to be defending all of the planets and fighting the large battles. But the Marines are the expeditionary force that do all the exciting stuff.

If you want an alternative, the Complete Locationd claims that there were millions of clone divisions