What is branching factor of mysql myisam's b-tree
There is about 8 bytes of overhead per index entry.  So, if the key is typically 6 bytes, and the key_buffer block size is 1KB, and the BTree is 69% full:
1024 * .69 / (8+6)

says there will be about 50 rows.
So, 3 levels of BTree can handle up to 50^3 = 125K rows.
The .69 comes from assuming lots of random inserts -- leading to each block oscillating between 100% full just before a block split and 50% after.
There may be an obscure SELECT against the information_schema to get the exact value for the indexes.  (I know of one for InnoDB.)