I would do something like
2024-01-01 opening balances
equity:start
;
; Several lots of Apple stock totaling 100 units of stock with average cost basis of $10
assets:brokerage1:aapl:2022-12-15 20 AAPL @ $8
assets:brokerage1:aapl:2023-01-20 30 AAPL @ $8
assets:brokerage1:aapl:2023-09-14 50 AAPL @ $12
;
; shares of a mutual fund 'XXX' totaling 50 units with average cost basis of $5
assets:brokerage1:xxx 50 XXX @ $5
(I think people usually track individual lots of stocks, but just average cost of mutual funds.)