A slot is a concurrently connected player, not a registered account. It affects two metrics at once, and both determine which plan is sufficient.
Memory grows linearly: a fixed amount in megabytes, specified by the title, is added to the base requirement of the server application per slot. The total is rounded up to half gigabytes — the value in the “Memory” column is exactly this total for standard slots.
Compute load grows flatter, but not linearly to the benefit: every slot above the first adds around 6% of the base value. The base value itself depends on the tick rate of the title — this is the number of game states the server calculates per second. 20 Hz results in a factor of one, double that means double the load. Therefore, a title with a high tick rate needs more machine with just a few players than a quiet sandbox with many.
The upper limit in the catalogue is 200 slots; for 20 of the listed titles, the panel also handles mod and plugin management.