Unless you got a rear cage you can get two in the rear window, up to 195/50x15. Four on passenger side with no seat. Two plus a bag of tools n jack if leave seat. When a strapped mine to bootlid it bent boot before it felt that sturdy tbh lol.
I used to put two in the passenger seat, one behind the passenger seat, and one in the boot along with a jack and some tools. Never needed more than four, really
This thread has definitely given me ideas about how to carry wheels/tyres once I get my mx5 on the road! I guess you could also modify/extend a normal boot rack to hold 2/4 securely too?
All about the boot rack, or if you have a roll bar and it's a sunny day (unlikely in the uk) lol sit them up against the roll bar and strap them to that tread facing forwards, leaves space on the boot rack too for more