This is Twinkling Titanite 40 00 00 00 00 00 04 6A
Search for this in your hex editor 400000000000046A (same as above, just no spaces)
After the 6A you will see 00 00 00 02. That is how many you have. Change the 02 to 63 and you will have 99 of them. When done it will look like this.
40 00 00 00 00 00 04 6A 00 00 00 63
You can also change what the item is if you want. Count over 8 numbers (bytes) from the beginning (40 00 00 00) and change the 00 00 04 6A to a different item if you want. If you change it to 00 00 04 4C you will have Red Titanite Slab and if you keep the 63 at the end of the string you will have 99 Red Titanite Slabs. So it will look like this...
40 00 00 00 00 00 04 4C 00 00 00 63
Hope this helps.