Hi I am relativly new to tropical fish and have only really been interested in reptiles before.

not sure whether this should go in the begginers corner.

how many guppys/platys could i fit in a sixty litre tank?

Is sand a safe substrate?

How many fry roughly are in each spawn for guppys?

I am intending to breed them in pretty short order

are there any breeding triggers that can help?

Are there any good books or articles you reccomend I read

I'd say two female guppies and two female platies, perhaps a male for each. Provided you have another tank for the fry to go into. Otherwise, have only males.

Depends what sand it is really. Most clean sand is fine.

I got about 20 fry each time fry my guppy but I've heard of 50+

They seem to breed on their own wether you like it or not.

HTH

Well Guppys and Platys will breed wherever and whenever lol! expect a new batch every 4-8 weeks.

Provide lots of hiding places for the fry (plants etc)

Lots of good water changes will encourage regular birthing .. 9 times out of 10 my females will birth straight after a water change as they can be known to hold on or reabsorb if the conditions aren't right.

water changes also encourages fast growth of the fry because there will be less nitrates.

Make sure you've got suitable food for tiny mouths.. i use decaps brine shrimp eggs which they love and then move onto crushed tropical flake and in no time they are eating what the adults eat.

My females are young so their drops are 7-20 but this will probably increase as they get older.

As Sam said, more females than males.