I had a question regarding salat. Once I have completed all my fardh prayers of the day, or if I am at a stage during the day where it isn’t time for the next salat , what should my niyya be and how many rakaa should I pray if I just feel like praying ?

After daily Wajib prayers there are another 23 Rakats which we call Nafila prayers.
In this concern there are 8 Rakats (2×4) Nafiala of Zohr which should be offered before Zohr Azan.
Same way there are another 8 (2×4) Rakats before Asr prayer.
After Maghrib there are 4 (2×2) Raka’at as a Nafila should be offered.
After Isha there are 2 Rakats which has to be offered while sitting.
There are 2 Rakats after Fajr prayer should be recited as a Nafila.
And there are 11 Rakats known as Salatul Layl.
So all these Nafila Namaz are Mustahab and have this flexibility to be offered other than it’s prescribed time in case someone has not offered them.