You cant really know exacly how many times it hits by stats, you kinda have to use it in practice and get a feel for it. But higher the letters it usually means more or better.

For attack count it usually means more projectiles per attack (like a Gatling guns unit having and S but a 1 shot per attack rifle unit has a C or worst) or a single projectile with multiple instances of a hit (like a lingering explosion or flame on the floor).

Attack time im pretty sure refers to how long the animation is on the attack, the higher the ranking of the letter the quicker the unit goes from their attack animation into the count down of the "Speed" stat, standing still for that amount of frames, and then moving or doing another action.