Depending on the size of the flask, you might get as few as 5-10 seedlings, or perhaps 20-50. You should ask the seller how many seedlings there are in the flask that you're thinking of getting.
The other thing to understand is that from seedling to a flowering plant, you would probably be looking at 3-5 years... so... if you're impatient for blooms like me, you might be better off just buying flowering-size plants
A quick google search on orchid flasks will lead you various sites that have some simple flasking guides, such as this one:
De-flasking and Community pots
Threaded Mode
