A start would be to simply do the sums to work out the added load m2 from those items you know you will be putting in. Without going into great detail AS1684 for light timber framing effectively uses ~40kg/m2 as a minimum for 'dead' loads (what you are adding) - and that includes all the loads being carried down to the sub-floor. That would be for regular 'light-weight' coverings such as carpet, vinyl etc. Adding heavier materials like tiles with an under-layer and so on would change the design specs needed. You are unlikely to cause any catastrophic changes, but if you exceed he design load by too much you can expect to see settling - and that will show as cracks and gaps in various places, doors not closing and so on.
When you do the sums come back and get some more advice - which will likely be more than just additional stumps, but could include doubling up of bearers (or extra rows) and maybe joists too. But estimates or guessing is not much use - get all the numbers, do the sums and come back.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.