Anaplan's list can handle only 999,999,999 items in it (post which it needs to be reset) . However Anaplan workspace size can not go more than 130 GB and every list item consumes 500 bytes (approx), now if you do the maths, you will be able to load less than 250-260 million items at once because you might hit 130 GB threshold.
Also the upload time into anaplan is directly proportional to the number of entries that you want to load. Just to give an idea it takes around 15-18 minutes to get the 15 million transactions (with couple of properties) loaded into anaplan
There's an index in the background that keeps track of all list member created, this index cannot exceed 999,999,999.
For example, if you have a list called "Transaction", and if at any time, you only have 1000 members, and you only have 1 module called "Transaction Module" with a number formatted line item called "Sales".
Then the total workspace consumed is about 1000 * 500b + 1000 * 8b = 0.508 Mb
Note: 1 list member takes up 500b per member, 1 number format takes up 8b.
Now, if you want to test the limit of how many concurrent list members you can have in Anaplan, then it's as @Misbah said, it's about 250-260 million member. The maths is as follow (assuming we also have 1 module with a number formatted line item using this list)