Is the capacity of a hash table a constant value?
There are rare cases where the number of entries in a hashtable is fixed. There are some more cases where the number of entries in one particular situation is known, and the right number of slots can be allocated before you start filling the hash table. 

But typical implementations start with a small number of slots, and if enough items are added so the expected number of collisions becomes too large, more slots are allocated and all items rearranged in the new slots. 

Usually the implementation tries to keep the percentage of used slots in some range, say (just an example) between 45% and 70%, compromising between efficiency and used space. So as you add items and use more than 70% of slots are used, the implementer would increase the number of slots so that now 45% are used, and more items can be added. An implementer might also shrink the table if items are removed and say less than 30% of slots are used, but in practice hash tables tend to grow.