There are limits to how many contacts you can keep in the system at one time. The “Max Number Of Contacts Allowed” is that limit. In your contacts area you will see the total amount of contacts you have in the system next to your limit.
If you reach that limit, you will need to remove some contacts before more can be imported to your system. To do that you can select the contacts you would like to remove on the contacts screen and in the “Actions” drop down select “Delete Contacts.”
Note: There is no daily limit of leads you can import. The only limit is the “Maximum Number Of Contact Allowed” amount of contacts your account can hold at one time. You can import contacts in batches over time, or all at once, as long as you stay under the maximum contact limit.