How many years of foreign tax credit I can carryforward to use in the future for foreign income when I return to live in the US?

the IRS only has the last 7 years of returns. There are many categories for foreign taxes. it's crucial to know the category because generally any FTC carryover can be used only against the same category of foreign income. for the 951A category, there is no carryover. for other categories, there is a 10-year carryover. however, if you used the foreign earned income exclusion there is another limitation on the amount that can be carried over.

You may have to reduce the foreign taxes you paid or accrued by the following items.
• Taxes on income excluded on Form 2555 (this is the general category). Reduce taxes paid or
accrued by the taxes allocable to any foreign earned income excluded on Form 2555. If only part of your foreign earned income is excluded, you must determine the amount of tax allocable to excluded income. To do so, multiply the foreign taxes paid or accrued on foreign earned income received or accrued during the tax year by the following fraction.
Numerator: Foreign earned income and housing amounts you excluded for the tax year minus otherwise deductible expenses (not including the foreign housing deduction) allocable to that income.
Denominator: Your total foreign earned income received or accrued during the tax year minus deductible expenses (including the foreign housing deduction) allocable to that income. However, if the foreign jurisdiction charges tax on foreign earned income and some other income (for example, earned income from U.S. sources or a type of income not subject to U.S. tax) and the taxes on the other income can't be segregated, the denominator is the total amount of income subject to foreign tax minus deductible expenses allocable to that income.
IRS PUB 514  provides further guidance

https://www.irs.gov/pub/irs-pdf/p514.pd