I am not exactly familiar with D&D, but I guess a web spell is supposed to prohibit the targets movement?
A block can either be on 1 target and block almost any action, or it can be on multiple targets blocking 1 specific action.
In that case, it would be a simple block on one target, and he wouldn't be able to do (almost) anything until he breaks free.
Or if you want to target an entire zone, you could block 1 action, in this case movement, and you would have to take 2 shifts of your spell to cover the whole zone.
so a Web (5 shifts block) could either be a 5 shift block on 1 target for 1 exchange or a 4 shift block for 2 exchanges or a 3 shift block to movement against 1 zone for 1 exchange or a 2 shift block on 1 zone for 2 exchanges. Or any other combination you can think of.
If a blocked target tries to make an action, he has to overcome the block value with his role, and the roll will still be lowered by the block value. However, once the block has been beaten, the block is no longer in effect. So if there are 3 people in your web, if one breaks free, the rest is free as well.