Matching the top CPM means that the campaign will be placed in the "first" slot : campaigns in this slot have the highest priority in displaying.
Anyways, this is no guarantee that a campaign is the only one in the slot, it is in fact possible to share a slot with one or more campaigns. In this case impressions will be shared with all these campaigns.
To be sure to get "all" the first impressions available the campaign has to be the only one in the slot.
I.e. if the TOP bid is currently 0.2, bidding 0.21 will get the campaign in the first slot, but it will likely share it with campaigns bidding 0.2.
The platform has a complex algorithm, many variables are involved and it is not possible to foresee what amount would trigger the creation of a new slot.
In this example, according to experience, bidding more than 0.25 would probably trigger the creation of a new slot with only that campaign.
Please be aware that TOP CPM of a campaign should not be considered as a static value: changes of the top bid will always be visible also to other advertisers, that might adjust their bids according to the new value.