The question is:
If scen rules allow a player to use some offboard troops as reinforcement that can enter map after playing a card, can a player divide it into some parts - for ex, enter 2 units at one turn and 2 more at another?

I would expect the special sceario rules to say how reinforcements are entered and these may be different for different scenarios. Is there a particular scenario you are thinking of?

The question appered during playing this one http://www.ccancients.net/Maps/Cimbrian-War-113-101-BC/206-aquae-sextiae-102-bc.html

It is not totally spelled out, but it says how "the force" enters play through an ambush event. So it suggests to me that all the units of the force all enter play together. If it talked about how units of the force may enter play it might mean they could enter seperately but by just talking about "the force" I am sure they all come on together.

But don't take my word for it. If you are still unsure contact GMT for an official reply. And if you do that please share the answer with us.

Hope this helps.