True polymorph is a spell that allows you to take any form you want and it states that the targets game statistics are replaced by the new form statistics including also its mental ability scores.
By "game statistics" its safe to assume that it means "anything and everything" that is written on the character sheet of the form assumed, including abilities spellcasting levels racial bonuses, even the spells should be the same as the original form taken, etc. (equipment shouldn't be replicated i believe).
If this is true then what happens with the spell slots you have used prior polymorphing to a new form? Are the new forms spell slots reduced by the exact number of spell slots used before turning to this new form or the new form has all its spell slots intact as this new body hasn't used its abilities yet?
From the spell description:
If this is read exactly as it is written, then the new form is the creature. For example, if you are a 17th level Wizard and cast the spell on yourself to make you a 17th level Cleric then you have all the statistics of a 17th level Cleric and none of the statistics of the 17th level Wizard.
Clearly these have exactly the same number of slots per day but different spells. You question is what happens to the slots expended by the wizard, for example, the one used to cast true polymorph?
First, it is not entirely clear and therefore subject to ruling by the DM.
However, it seems to me that the only sensible option is that the "new form" is exactly that, a new form with all spell slots available. To rule otherwise invites madness, what if instead of originally being a full spell caster like a wizard, it was a semi-spell caster like a ranger - or the transformation went from wizard to ranger. Don't even think about how to handle a warlock!
This is the simplest and cleanest solution, anything else invites so many exceptions, corner cases and "But what if ... ?" that it would become unmanageable.
There is an argument that this interpretation makes the spell very powerful; to which I say: