Probably worth starting with the current squad, based upon CL rules for Club trained (C), Other FA Affiliated Clubs (HG) and u21's :
GKs - Lloris, Gazzaniga, Vorm (out of contract)
CB's - Alderweireld, Vertonghen, Sanchez, Foyth (u21 in 2019/20 for CL)
FB's - Rose (C), Davies, Aurier, Trippier (HG), Walker-Peters (C)
CM's - Wanyama, Dier, Winks (C), Sissoko, (plus Amos (C), Skipp (u21)
AM's - Eriksen, Son, Dele (HG), Moura, Lamela, N'Koudou
FWD's - Kane (C), Llorente, Janssen
Assuming Vorm leaves and with Foyth having been with Spurs for 2 years so qualifying as an u21 for 2019/20 (after that he will become an Overseas trained player), we are actually at 17 Overseas Trained players (our maximum).
We have 5 Club trained players (Kane, Rose, Winks, Walker-Peters and Amos) and players trained by other clubs affiliated with the FA (Trippier, Dele) - so space for one more, or two if Amos is loaned out.
But to create space for incoming overseas trained players, we need to assume N'Koudou and Janssen are sold.
1, We need a 3rd GK which could be either Alfie Whiteman from the academy or if another brought in preferably a Club Trained GK (eg Button) or other home grown player so as not to take an overseas trained player. That assumes Vorm's contract will not be renewed,
2. Wanyama appears to be injured beyond repair, whilst Lamela has been significantly injured for all but one of his 5 years at Spurs, so neither can be relied upon and need to leave to be replaced by players who can be relied upon to be more fit.
3. Serge Aurier has not impressed much over his 2 seasons at Spurs - time to move him on to generate fees and get in an upgrade. His RB colleague Kieran Trippier started the season well, but has looked vulnerable since against the better sides - with HG status, I'd move Aurier on before Trippier.
4. Fernando Llorente has done well in infrequent appearances, but is not able to play multiple games in a row and as his contract is up, I'd look to get in another Striker/AM rather than renewing Llorente's contract for another year.
5. Up to Poch whether he can bring through Luke Amos (sadly injured for the season very early on) and Oliver Skipp at the same time - and not forgetting Jamie Bowden who has been 19th man on a couple of occasions. It may be that Amos goes on loan whilst bringing in a more experienced CM (in effect replacing Dembele),
I don't see us bringing in more than 4-6 first team players, its just too difficult to not only identify and bring them in, but critically to also integrate them, so that's probably it !
But the big questions must be over Alderweireld, Eriksen and Vertonghen all of whose contracts have only one more year to run. With the new stadium likely to allow a much higher wage structure and Spurs being in CL finals rather than the likes of Barcelona, Real Madrid and Manchester United, I am hoping we are able to persuade them all that Spurs is the place to be for the next few years. But if not we need to add them to the list !