You need a separate resistor for each LED.
A very simplified way of thinking about this is like this…
If you had 10 LED-resistor combinations, you would take 200 mA from the battery.
If you took one of these resistors and connected it to 10 LED’s in parallel you would only take the same 20 mA that the one LED would consume, but this would cause all the LED’s to be very dim because they would only have an average of 2 mA each ( but as a previous member mentioned this would not be shared equally due to tiny differences in the Vf of each LED ).
If you swapped the resistor for one that would take 200 mA from the battery, then in theory you would have an " average " of 20 mA for each LED, but this also would not be shared equally.
The LED with the lowest Vf ( needs to only be a couple of milli-volts lower ) will take more than its share of current and will burn out long before its rated lifespan; this could be as short as a few minutes ( seconds if you are using cheap LED’s with a large difference in Vf ) or may last days or even weeks.
Your 200 mA is now shared between 9 LED’s, so the LED with the next lowest Vf gets more punishment than its predecessor and burns out even quicker.
Repeat this until all of your LED’s are cooked !
Resistors will only cost you pennies, and leaving them out is not worth the risk.