There are 4,416 cities on the planet with 150k+ population. 10 100kt bombs are more than enough to destroy most cities on the planet, at least when you define "destroy" as "most everything is broken and/or on fire and most people are dead within 1 year's time".
After your 10-nukes-per-city campaign, you've still got another 5,840 100kt bombs left in your 5,000 megaton budget to divide among the largest cities (NYC, Tokyo, Hong Kong, Tehran, etc.).
I'm stating this because I don't think it's too useful to predict the secondary effects (nuclear winter, etc.) of a war so terrible that basically everyone on the planet would already be dead (from blast, fire, etc.) or dying within a year's time (radiation, starvation from infrastructure collapse, etc.).