The leaked database dumps (from when MtGox was hacked after they closed down) shows 189374 records as a result for this query:

mysql> SELECT * FROM platform.User_Wallet WHERE platform.User_Wallet.Balance != 0 ORDER BY platform.User_Wallet.Balance DESC;

Notes:

• That's only non-zero balances
• There are many currencies (BTC, USD, JPY, PLN, AUD, GBP, ...) in there, each counting as a seperate "wallet"
• We don't know how many fake accounts there are (created by MtGox themselves).
• The hackers that released these documents may have messed with them.
• There are for example 8467 records that have Balance "1" (Satoshi or other currency). Likely remainders of incomplete withdrawals and such. There will be many many more that have up to a couple million Satoshis (which is still just dust).

Therefore, in short, this may NOT be a good approximate answer to your exact question.