About 6 on their own (no other tank mates). They are schooling fish so they do need this sort of group.
They mix very well with platies and other peaceful fish.
If you realy want to keep both in a 10 gallon, you would need to consider that platies are a livebearer so you must keep a ratio of 2 females to every male or stick to just one sex (prefferavly male as then you wont get over-stocked by fry from stored sperm) and you must also consider that the rasboras need to be kept in a group to thrive (prefferably a group of 6 but a 10 gallon would be barely enough for just these).
As such, your only real option in a 10 gallon would be 2 male platies and 4 harlequins. No other fish could be added without a risk of over-stocking.